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, 6 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index 6b07e24..1e98351 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -5,11 +5,13 @@ import type { EventEmitter, RunTurnInput, RunTurnResult } from "../contracts/run
 import type { ToolCall, ToolContract } from "../contracts/tool.js";
 import { createStepDispatcher, type StepDispatcher } from "./dispatch.js";
 import {
+	doneEvent,
 	errorEvent,
 	reasoningDeltaEvent,
 	textDeltaEvent,
 	toolCallEvent,
 	toolResultEvent,
+	turnStartEvent,
 	usageEvent,
 } from "./events.js";
 
@@ -346,6 +348,8 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 	// Track open tool-call spans across steps so we can close them on abort
 	const toolSpans = new Map<string, Span>();
 
+	input.emit(turnStartEvent(conversationId, turnId));
+
 	try {
 		for (let step = 0; step < MAX_STEPS; step++) {
 			if (signal.aborted) {
@@ -422,6 +426,8 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 		}
 	}
 
+	input.emit(doneEvent(conversationId, turnId, finishReason));
+
 	return { messages: resultMessages, usage: totalUsage, finishReason };
 }