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/journal/src/main/kotlin/com/correx/core/journal/DefaultDecisionJournalReducer.kt

@@ -2,6 +2,7 @@ package com.correx.core.journal
 
 import com.correx.core.events.events.ApprovalDecisionResolvedEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
+import com.correx.core.events.events.ExecutionPlanLockedEvent
 import com.correx.core.events.events.InitialIntentEvent
 import com.correx.core.events.events.RetryAttemptedEvent
 import com.correx.core.events.events.StageFailedEvent
@@ -14,6 +15,7 @@ import com.correx.core.journal.model.DecisionRecord
 
 class DefaultDecisionJournalReducer : DecisionJournalReducer {
     override fun reduce(state: DecisionJournalState, event: StoredEvent): DecisionJournalState {
+        if (event.payload is ExecutionPlanLockedEvent) return state.copy(planLocked = true)
         val record = when (val p = event.payload) {
             is InitialIntentEvent -> DecisionRecord(
                 sequence = event.sessionSequence,
@@ -22,8 +24,9 @@ class DefaultDecisionJournalReducer : DecisionJournalReducer {
             )
             is SteeringNoteAddedEvent -> DecisionRecord(
                 sequence = event.sessionSequence,
-                kind = DecisionKind.STEERING,
-                summary = "User steering: ${p.content}",
+                kind = if (state.planLocked) DecisionKind.PREEMPT else DecisionKind.STEERING,
+                summary = if (state.planLocked) "PRIORITY DIRECTIVE (overrides plan): ${p.content}"
+                          else "User steering: ${p.content}",
                 stageId = p.stageId?.value,
             )
             is ApprovalDecisionResolvedEvent -> DecisionRecord(

core/journal/src/main/kotlin/com/correx/core/journal/model/DecisionJournalState.kt

@@ -5,4 +5,5 @@ import kotlinx.serialization.Serializable
 @Serializable
 data class DecisionJournalState(
     val records: List<DecisionRecord> = emptyList(),
+    val planLocked: Boolean = false,
 )

core/journal/src/main/kotlin/com/correx/core/journal/model/DecisionRecord.kt

@@ -2,7 +2,7 @@ package com.correx.core.journal.model
 
 import kotlinx.serialization.Serializable
 
-enum class DecisionKind { INTENT, STEERING, APPROVAL, TRANSITION, RETRY, FAILURE, ARTIFACT }
+enum class DecisionKind { INTENT, STEERING, PREEMPT, APPROVAL, TRANSITION, RETRY, FAILURE, ARTIFACT }
 
 /**
  * One distilled decision. [sequence] is the source event's sessionSequence, used for