feat(context): reasoning-thread continuity, L3 doc filtering, and gate hardening

Threads reasoning_content across inference calls and journal renders, filters
markdown noise out of L3 repo-knowledge retrieval, adds PlanLinter H3 checks,
and tightens filesystem tool output/dir-listing behavior surfaced by prior
live QA (see project_readloop_campaign memory).

core/events/src/main/kotlin/com/correx/core/events/events/RepoKnowledgeEvents.kt

@@ -25,3 +25,18 @@ data class RepoKnowledgeRetrievedEvent(
     val query: String,
     val hits: List<RepoKnowledgeHit>,
 ) : EventPayload
+
+/**
+ * Candidates that scored above the L3 index's own top-k cutoff but below the retriever's
+ * minimum-similarity floor, so were never rendered to the agent as "relevant." Without this,
+ * a hit below the floor is indistinguishable from one that was never a candidate — recorded
+ * per invariant #9 since the embedding-similarity score is itself an environment observation.
+ */
+@Serializable
+@SerialName("RepoKnowledgeHitsFiltered")
+data class RepoKnowledgeHitsFilteredEvent(
+    val sessionId: SessionId,
+    val query: String,
+    val threshold: Float,
+    val dropped: List<RepoKnowledgeHit>,
+) : EventPayload

core/events/src/main/kotlin/com/correx/core/events/serialization/Serialization.kt

@@ -57,6 +57,7 @@ import com.correx.core.events.events.ModelUnloadedEvent
 import com.correx.core.events.events.OrchestrationPausedEvent
 import com.correx.core.events.events.OrchestrationResumedEvent
 import com.correx.core.events.events.RefinementIterationEvent
+import com.correx.core.events.events.RepoKnowledgeHitsFilteredEvent
 import com.correx.core.events.events.RepoKnowledgeRetrievedEvent
 import com.correx.core.events.events.RepoMapComputedEvent
 import com.correx.core.events.events.RetryAttemptedEvent
@@ -154,6 +155,7 @@ val eventModule = SerializersModule {
         subclass(RepoMapComputedEvent::class)
         subclass(WorkspaceStateObservedEvent::class)
         subclass(RepoKnowledgeRetrievedEvent::class)
+        subclass(RepoKnowledgeHitsFilteredEvent::class)
         subclass(BriefGroundingCheckedEvent::class)
         subclass(BriefEchoMismatchEvent::class)
         subclass(StaticAnalysisCompletedEvent::class)