feat(kernel-runtime,session-orchestrator): emit turn lifecycle events

Close a gap found live: neither transport emitted turn-start/done/turn-sealed
(the wire defined them; nothing fired them). turn-sealed is the FE's
cache-commit signal (frontend-design §6.3); done ends the stream.

- kernel-runtime: runTurn emits turn-start first and done (with finishReason)
  last, on every exit path (stop/tool-calls/max-steps/error/aborted).
- session-orchestrator: emits turn-sealed after conversationStore.append
  succeeds (the kernel touches no DB, so the post-persist seal is the
  orchestrator's). Not emitted if append throws.

No contract change (all three wire types already existed). Verified live: HTTP
/chat and WS chat both stream turn-start … done turn-sealed.

typecheck clean, 494 vitest + 80 bun, biome clean.

1 files changed, 8 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/events.ts b/packages/kernel/src/runtime/events.ts
index 2a92008..a209b00 100644
--- a/packages/kernel/src/runtime/events.ts
+++ b/packages/kernel/src/runtime/events.ts
@@ -48,6 +48,14 @@ export function usageEvent(conversationId: string, turnId: string, usage: Usage)
 	return { type: "usage", conversationId, turnId, usage };
 }
 
+export function turnStartEvent(conversationId: string, turnId: string): AgentEvent {
+	return { type: "turn-start", conversationId, turnId };
+}
+
+export function doneEvent(conversationId: string, turnId: string, reason: string): AgentEvent {
+	return { type: "done", conversationId, turnId, reason };
+}
+
 export function errorEvent(
 	conversationId: string,
 	turnId: string,