feat(tasks): duplicate-title guard on task creation

The doctrine says "search before creating", but that leans on the LLM; this is
the backstop against an agent re-creating a task across runs. TaskService.
findDuplicates matches active (non-terminal) same-title tasks, case- and
whitespace-insensitively — a DONE/CANCELLED look-alike is not a duplicate, since
that work may legitimately recur.

- task_create rejects a duplicate, naming the look-alike and offering force=true.
- POST /tasks → 409 with the duplicates, or force:true to override;
  correx task create --force.

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

apps/cli/src/main/kotlin/com/correx/apps/cli/commands/TaskCommand.kt

@@ -6,6 +6,7 @@ import com.github.ajalt.clikt.core.CliktCommand
 import com.github.ajalt.clikt.core.subcommands
 import com.github.ajalt.clikt.parameters.arguments.argument
 import com.github.ajalt.clikt.parameters.options.default
+import com.github.ajalt.clikt.parameters.options.flag
 import com.github.ajalt.clikt.parameters.options.multiple
 import com.github.ajalt.clikt.parameters.options.option
 import com.github.ajalt.clikt.parameters.options.required
@@ -206,6 +207,7 @@ class TaskCreateCommand : TaskHttpCommand("create") {
     private val goal by option("--goal").required()
     private val criteria by option("--criteria", help = "Acceptance criterion (repeatable)").multiple()
     private val paths by option("--path", help = "Affected path/glob (repeatable)").multiple()
+    private val force by option("--force", help = "Create even if a same-title task exists").flag()
 
     override fun run() = http { client ->
         val payload = buildJsonObject {
@@ -214,12 +216,19 @@ class TaskCreateCommand : TaskHttpCommand("create") {
             put("goal", goal)
             if (criteria.isNotEmpty()) put("acceptanceCriteria", JsonArray(criteria.map { JsonPrimitive(it) }))
             if (paths.isNotEmpty()) put("affectedPaths", JsonArray(paths.map { JsonPrimitive(it) }))
+            if (force) put("force", true)
         }
-        val raw = client.post("${baseUrl()}/tasks") {
+        val resp = client.post("${baseUrl()}/tasks") {
             contentType(ContentType.Application.Json)
             setBody(payload)
-        }.bodyAsText()
-        if (jsonOut()) println(raw) else println("created ${taskJson.decodeFromString<TaskRow>(raw).id}")
+        }
+        val raw = resp.bodyAsText()
+        when {
+            resp.status == HttpStatusCode.Conflict ->
+                System.err.println("Possible duplicate(s) — pass --force to create anyway:\n${renderTaskList(taskJson.decodeFromString(raw))}")
+            jsonOut() -> println(raw)
+            else -> println("created ${taskJson.decodeFromString<TaskRow>(raw).id}")
+        }
     }
 }