feat(tasks): per-task history / audit timeline

Surfaces a task's lifecycle straight from the event log (the source of truth) so
you can see what actually happened to it — claim/submit/complete, notes, links,
git-driven status — rather than only its current state. Useful for debugging a
live agent run, where current state alone doesn't say how it got there.

- TaskHistory.render: pure timeline renderer (split content/lifecycle to stay under
  the complexity cap, mirroring DefaultTaskReducer).
- TaskService.history(id): the task's own events, in order; survives soft-delete
  (the deletion is part of the trail), empty for an id that never existed.
- GET /tasks/{id}/history (200 text timeline, 404 when the id never existed) and
  the `correx task history <id>` CLI command.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/tasks/src/main/kotlin/com/correx/core/tasks/TaskHistory.kt

@@ -0,0 +1,64 @@
+package com.correx.core.tasks
+
+import com.correx.core.events.events.StoredEvent
+import com.correx.core.events.events.TaskAcceptanceCriteriaSetEvent
+import com.correx.core.events.events.TaskAffectedPathsSetEvent
+import com.correx.core.events.events.TaskBlockedEvent
+import com.correx.core.events.events.TaskCancelledEvent
+import com.correx.core.events.events.TaskClaimedEvent
+import com.correx.core.events.events.TaskCompletedEvent
+import com.correx.core.events.events.TaskCreatedEvent
+import com.correx.core.events.events.TaskDeletedEvent
+import com.correx.core.events.events.TaskEditedEvent
+import com.correx.core.events.events.TaskEvent
+import com.correx.core.events.events.TaskLinkedEvent
+import com.correx.core.events.events.TaskNoteAddedEvent
+import com.correx.core.events.events.TaskReleasedEvent
+import com.correx.core.events.events.TaskReopenedEvent
+import com.correx.core.events.events.TaskSubmittedForReviewEvent
+import com.correx.core.events.events.TaskUnblockedEvent
+import com.correx.core.events.events.TaskUnlinkedEvent
+
+/**
+ * Renders a task's event log as a human-readable lifecycle timeline. The log is the source of
+ * truth, so this is the audit trail straight from the facts — one line per event, in order,
+ * `<timestamp>  <what happened>`. Useful for seeing what an agent actually did to a task
+ * (claim/submit/complete, notes, links, git-driven status) rather than just its current state.
+ */
+object TaskHistory {
+
+    fun render(events: List<StoredEvent>): String {
+        val lines = events.mapNotNull { e ->
+            val payload = e.payload as? TaskEvent ?: return@mapNotNull null
+            "${e.metadata.timestamp}  ${describe(payload)}"
+        }
+        return if (lines.isEmpty()) "no history" else lines.joinToString("\n")
+    }
+
+    private fun describe(p: TaskEvent): String =
+        describeContent(p) ?: describeLifecycle(p) ?: (p::class.simpleName ?: "event")
+
+    private fun describeContent(p: TaskEvent): String? = when (p) {
+        is TaskCreatedEvent -> "created ${p.key}: ${p.title}"
+        is TaskEditedEvent -> "edited"
+        is TaskAcceptanceCriteriaSetEvent -> "acceptance criteria set (${p.criteria.size})"
+        is TaskAffectedPathsSetEvent -> "affected paths set (${p.paths.size})"
+        is TaskLinkedEvent -> "linked ${p.type} -> ${p.targetId} (${p.targetKind})"
+        is TaskUnlinkedEvent -> "unlinked ${p.type} -> ${p.targetId}"
+        is TaskNoteAddedEvent -> "note [${p.author}] ${p.body}"
+        is TaskDeletedEvent -> "deleted"
+        else -> null
+    }
+
+    private fun describeLifecycle(p: TaskEvent): String? = when (p) {
+        is TaskClaimedEvent -> "claimed by ${p.claimant}"
+        is TaskReleasedEvent -> "released"
+        is TaskBlockedEvent -> "blocked: ${p.reason}"
+        is TaskUnblockedEvent -> "unblocked"
+        is TaskSubmittedForReviewEvent -> "submitted for review"
+        is TaskCompletedEvent -> "completed"
+        is TaskReopenedEvent -> "reopened"
+        is TaskCancelledEvent -> "cancelled" + (p.reason?.let { ": $it" } ?: "")
+        else -> null
+    }
+}

core/tasks/src/main/kotlin/com/correx/core/tasks/TaskService.kt

@@ -3,6 +3,7 @@ package com.correx.core.tasks
 import com.correx.core.events.events.EventMetadata
 import com.correx.core.events.events.EventPayload
 import com.correx.core.events.events.NewEvent
+import com.correx.core.events.events.StoredEvent
 import com.correx.core.events.events.TaskAcceptanceCriteriaSetEvent
 import com.correx.core.events.events.TaskAffectedPathsSetEvent
 import com.correx.core.events.events.TaskBlockedEvent
@@ -12,6 +13,7 @@ import com.correx.core.events.events.TaskCompletedEvent
 import com.correx.core.events.events.TaskCreatedEvent
 import com.correx.core.events.events.TaskDeletedEvent
 import com.correx.core.events.events.TaskEditedEvent
+import com.correx.core.events.events.TaskEvent
 import com.correx.core.events.events.TaskLinkedEvent
 import com.correx.core.events.events.TaskNoteAddedEvent
 import com.correx.core.events.events.TaskReleasedEvent
@@ -70,6 +72,14 @@ class TaskService(
         return if (state.deleted) null else Task(taskId, state)
     }
 
+    /**
+     * A task's own events in order — the lifecycle audit trail straight from the log (survives
+     * soft-delete, unlike [getTask]). Empty when the task never existed. Render with [TaskHistory].
+     */
+    fun history(taskId: TaskId): List<StoredEvent> =
+        eventStore.read(TaskStreams.forProject(projectOf(taskId)))
+            .filter { (it.payload as? TaskEvent)?.taskId == taskId }
+
     /** Ranked text search over one project (when given) or the whole cross-project board. */
     fun search(query: String, projectId: ProjectId? = null): List<Task> =
         TaskSearch.search(projectId?.let { list(it) } ?: listAll(), query)

core/tasks/src/test/kotlin/com/correx/core/tasks/TaskHistoryTest.kt

@@ -0,0 +1,81 @@
+package com.correx.core.tasks
+
+import com.correx.core.events.events.EventMetadata
+import com.correx.core.events.events.EventPayload
+import com.correx.core.events.events.StoredEvent
+import com.correx.core.events.events.TaskCancelledEvent
+import com.correx.core.events.events.TaskClaimedEvent
+import com.correx.core.events.events.TaskCompletedEvent
+import com.correx.core.events.events.TaskCreatedEvent
+import com.correx.core.events.events.TaskNoteAddedEvent
+import com.correx.core.events.events.TaskSubmittedForReviewEvent
+import com.correx.core.events.types.EventId
+import com.correx.core.events.types.ProjectId
+import com.correx.core.events.types.SessionId
+import com.correx.core.events.types.TaskId
+import com.correx.core.events.types.TaskNoteAuthor
+import kotlinx.datetime.Instant
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class TaskHistoryTest {
+
+    private val taskId = TaskId("auth-1")
+
+    private fun stored(payload: EventPayload, seq: Long) = StoredEvent(
+        metadata = EventMetadata(
+            eventId = EventId("e-$seq"),
+            sessionId = SessionId("tasks:auth"),
+            timestamp = Instant.parse("2026-01-0${seq}T00:00:00Z"),
+            schemaVersion = 1,
+            causationId = null,
+            correlationId = null,
+        ),
+        sequence = seq,
+        sessionSequence = seq,
+        payload = payload,
+    )
+
+    private fun created() = TaskCreatedEvent(
+        taskId = taskId,
+        projectId = ProjectId("auth"),
+        key = "auth-1",
+        title = "Add JWT refresh",
+        goal = "users stay authenticated",
+    )
+
+    @Test
+    fun `renders the lifecycle as a timeline in order`() {
+        val events = listOf(
+            stored(created(), 1),
+            stored(TaskClaimedEvent(taskId, "claude-opus"), 2),
+            stored(TaskNoteAddedEvent(taskId, TaskNoteAuthor.AGENT, "migration retried"), 3),
+            stored(TaskSubmittedForReviewEvent(taskId), 4),
+            stored(TaskCompletedEvent(taskId), 5),
+        )
+
+        val lines = TaskHistory.render(events).lines()
+
+        assertEquals(5, lines.size)
+        assertTrue(lines[0].contains("2026-01-01T00:00:00Z"), lines[0])
+        assertTrue(lines[0].contains("created auth-1: Add JWT refresh"), lines[0])
+        assertTrue(lines[1].contains("claimed by claude-opus"), lines[1])
+        assertTrue(lines[2].contains("note [AGENT] migration retried"), lines[2])
+        assertTrue(lines[3].contains("submitted for review"), lines[3])
+        assertTrue(lines[4].contains("completed"), lines[4])
+    }
+
+    @Test
+    fun `cancellation reason is included`() {
+        val lines = TaskHistory.render(
+            listOf(stored(created(), 1), stored(TaskCancelledEvent(taskId, "superseded"), 2)),
+        ).lines()
+        assertTrue(lines[1].contains("cancelled: superseded"), lines[1])
+    }
+
+    @Test
+    fun `empty event list renders as no history`() {
+        assertEquals("no history", TaskHistory.render(emptyList()))
+    }
+}

core/tasks/src/test/kotlin/com/correx/core/tasks/TaskServiceTest.kt

@@ -89,4 +89,30 @@ class TaskServiceTest {
         // count() is creation-based, so the deleted id is never reused.
         assertEquals("auth-2", third.taskId.value)
     }
+
+    @Test
+    fun `history returns the task's own events in order`() = runBlocking {
+        val id = service.createTask(project, "Add JWT refresh", "auth").taskId
+        service.claim(id, "claude-opus")
+        service.submitForReview(id)
+        service.complete(id)
+        // A second task in the same stream must not bleed into this one's history.
+        service.createTask(project, "unrelated", "g")
+
+        val lines = TaskHistory.render(service.history(id)).lines()
+        assertEquals(4, lines.size)
+        assertTrue(lines.first().contains("created"))
+        assertTrue(lines.last().contains("completed"))
+    }
+
+    @Test
+    fun `history survives soft-delete but is empty for an unknown task`() = runBlocking {
+        val id = service.createTask(project, "throwaway", "oops").taskId
+        service.delete(id)
+
+        // getTask hides a deleted task, but its audit trail (incl. the deletion) remains.
+        assertNull(service.getTask(id))
+        assertTrue(service.history(id).isNotEmpty())
+        assertTrue(service.history(TaskId("auth-999")).isEmpty())
+    }
 }