feat(router): enrich L2 decision points with steering and failure reason

Fix broken stage-summary interpolation ($payload.stageId rendered the event's toString()). RouterL2Entry gains reason + steeringNotes; the reducer folds SteeringNoteAddedEvent into pendingSteeringNotes and harvests them into the completing stage's L2 entry, then clears (adr-0003 §4, pure field extraction, zero inference). RouterContextBuilder renders summary + reason + steering.

testing/deterministic/src/test/kotlin/RouterProjectorTest.kt

@@ -147,13 +147,13 @@ class RouterProjectorTest {
     }
 
     @Test
-    fun `apply(SteeringNoteAddedEvent) leaves state unchanged`() {
+    fun `apply(SteeringNoteAddedEvent) accumulates the note into pendingSteeringNotes`() {
         val state = projector.initial()
         val updated = projector.apply(
             state,
             stored(payload = SteeringNoteAddedEvent(sessionId, "do something different")),
         )
-        assertEquals(state, updated)
+        assertEquals(listOf("do something different"), updated.pendingSteeringNotes)
     }
 
     @Test

testing/deterministic/src/test/kotlin/RouterReducerTest.kt

@@ -155,7 +155,7 @@ class RouterReducerTest {
         val entry = updated.l2Memory[0]
         assertEquals(stageId, entry.stageId)
         assertEquals(StageOutcomeKind.FAILURE, entry.outcome)
-        assertTrue(entry.summary.contains("timeout"))
+        assertEquals("timeout", entry.reason)
         assertNull(updated.currentStageId)
     }
 
@@ -175,18 +175,18 @@ class RouterReducerTest {
     }
 
     @Test
-    fun `SteeringNoteAddedEvent does not change state`() {
+    fun `SteeringNoteAddedEvent accumulates into pendingSteeringNotes without touching conversation`() {
         val state = reducer.initial
         val updated = reducer.reduce(
             state,
             stored(payload = SteeringNoteAddedEvent(sessionId, "do something different")),
         )
-        assertEquals(state, updated)
+        assertEquals(listOf("do something different"), updated.pendingSteeringNotes)
         assertTrue(updated.conversationHistory.isEmpty())
     }
 
     @Test
-    fun `SteeringNoteAddedEvent on non-initial state does not change state`() {
+    fun `SteeringNoteAddedEvent preserves workflow status and current stage`() {
         val initial = reducer.initial
         val started = reducer.reduce(
             initial,
@@ -196,11 +196,22 @@ class RouterReducerTest {
             started,
             stored(payload = SteeringNoteAddedEvent(sessionId, "shift focus")),
         )
-        assertEquals(started, withNote)
+        assertEquals(listOf("shift focus"), withNote.pendingSteeringNotes)
         assertEquals(WorkflowStatus.RUNNING, withNote.workflowStatus)
         assertEquals(stageId, withNote.currentStageId)
     }
 
+    @Test
+    fun `steering notes are harvested into the completing stage's L2 entry and then cleared`() {
+        val initial = reducer.initial
+        val started = reducer.reduce(initial, stored(payload = WorkflowStartedEvent(sessionId, workflowId, stageId)))
+        val steered = reducer.reduce(started, stored(payload = SteeringNoteAddedEvent(sessionId, "call it the runtime")))
+        val completed = reducer.reduce(steered, stored(payload = StageCompletedEvent(sessionId, stageId, StageId("t1"))))
+
+        assertEquals(listOf("call it the runtime"), completed.l2Memory[0].steeringNotes)
+        assertTrue(completed.pendingSteeringNotes.isEmpty())
+    }
+
     @Test
     fun `unknown event type returns unchanged state`() {
         val state = reducer.initial