fix: complete P4 audit findings — steering wiring, generation config

P4-1: Wire steering to actually work (advisory-only):
  - RouterFacade emits SteeringNoteAddedEvent via event store in STEERING mode
  - SessionOrchestrator reads steering notes from event store and includes them
    as ContextEntry objects (sourceType=steeringNote) in stage context packs
  - Delete dead SteeringNote data class; single vocabulary = SteeringNoteAddedEvent
  - Add optional validateSteering callback to DefaultRouterFacade for future
    ValidationPipeline wiring (avoids cross-core dependency)
  - Update RouterFacadeTest to assert event emission

P4-2: Router FACE — move hardcoded generation config:
  - Add GenerationConfig field to RouterConfig with same defaults
  - RouterFacade uses config.generationConfig instead of inline literals
  - (Remaining P4-2 items: conversation persistence and coordination
    semantics are design decisions requiring ADRs, deferred)

core/context/src/main/kotlin/com/correx/core/context/steering/SteeringNote.kt

@@ -1,15 +0,0 @@
-package com.correx.core.context.steering
-
-import com.correx.core.events.types.SessionId
-import com.correx.core.events.types.StageId
-import kotlinx.datetime.Clock
-import kotlinx.datetime.Instant
-import kotlinx.serialization.Serializable
-
-@Serializable
-data class SteeringNote(
-    val sessionId: SessionId,
-    val content: String,
-    val stageId: StageId? = null,
-    val createdAt: Instant = Clock.System.now(),
-)