feat(freestyle): soft-lock steering preemption (Slice 5)

- DecisionKind gains PREEMPT; DecisionJournalState tracks planLocked.
- DefaultDecisionJournalReducer: ExecutionPlanLockedEvent flips planLocked
  (no record); post-lock steering notes recorded as PREEMPT with a
  'PRIORITY DIRECTIVE' summary, pre-lock stay STEERING.
- SessionOrchestrator.buildSteeringNoteEntries: post-lock notes surfaced as
  pinned L0/SYSTEM 'PRIORITY DIRECTIVE' entries ahead of the plan; pre-lock
  keep L2. Graph re-routing intentionally deferred (priority surfacing only).
- Tests: PreemptSteeringJournalTest, SteeringPreemptionTest.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -38,6 +38,7 @@ import com.correx.core.events.events.NewEvent
 import com.correx.core.events.events.OrchestrationPausedEvent
 import com.correx.core.events.events.OrchestrationResumedEvent
 import com.correx.core.events.events.RiskAssessedEvent
+import com.correx.core.events.events.ExecutionPlanLockedEvent
 import com.correx.core.events.events.SteeringNoteAddedEvent
 import com.correx.core.events.events.ToolExecutionRejectedEvent
 import com.correx.core.events.events.ToolInvocationRequestedEvent
@@ -787,12 +788,15 @@ abstract class SessionOrchestrator(
 
     private suspend fun buildSteeringNoteEntries(sessionId: SessionId): List<ContextEntry> {
         val events = eventStore.read(sessionId)
+        var locked = false
         return events.mapNotNull { event ->
             when (val p = event.payload) {
+                is ExecutionPlanLockedEvent -> { locked = true; null }
                 is SteeringNoteAddedEvent -> ContextEntry(
                     id = ContextEntryId(UUID.randomUUID().toString()),
-                    layer = ContextLayer.L2,
-                    content = p.content,
+                    layer = if (locked) ContextLayer.L0 else ContextLayer.L2,
+                    content = if (locked) "PRIORITY DIRECTIVE — overrides the locked plan: ${p.content}"
+                              else p.content,
                     sourceType = "steeringNote",
                     sourceId = p.stageId?.value ?: sessionId.value,
                     tokenEstimate = estimateTokens(p.content),