Turn services + agent DI: chat-handler becomes the composition root (stage 4)

The chat turn is now composed, not implemented, by chat-handler.ts:

- src/server/turn-service.ts: one named, error-contained seam per turn
  responsibility — buildTeacherContext (W8 read channel), persistTurn
  (canonical state write), spawnTurnJobs (async protocol kick-off),
  foldTurnIntoGraph (session→graph consolidation; the extraction-pipeline
  stage swaps this body behind the same seam). Each takes its deps
  explicitly; failures log one [chat <id>] line and never touch the
  response.
- chat-handler.ts wires: storage ports (stores), the job runner, the
  teacher tools (ctx = chatId + injected ChatStore), and the services.
  It no longer contains any logic beyond request validation.

Agent layer now receives dependencies instead of importing them — a new
lint zone enforces it (src/agent never imports src/server or src/store;
verified to fire both ways):

- runner.ts: createJobRunner({ chatStore, jobStore }) factory returning
  the spawn hook + the three job runners; the model-override test seam
  stays module-level. Bodies otherwise verbatim.
- plan tools: persist through the ctx-injected ChatStore
  (TeacherToolContext = { chatId, chatStore }).
- teacher-agent's dependency-free default registry is now the true
  client-side set: questionTools + jobTools (jobTools was ctx-parameter-
  shaped but never used its ctx — now honestly context-free). Production
  always passes createTeacherTools({ chatId, chatStore }); tests that
  drive a bare mock model get the client-side set unchanged.

Zero behavior change: tsc clean, 190/190 tests, eslint clean, production
build clean. Tests inject the same fs ports the chat-handler does.

src/agent/tools.ts

@@ -1,6 +1,9 @@
 import { tool, type ToolSet } from "ai";
 import { askInputSchema, quizInputSchema } from "../core/question-core.ts";
+import type { ChatStore } from "../core/ports.ts";
 import { jobTools } from "./tools/jobs.ts";
+
+export { jobTools };
 import { planTools } from "./tools/plan.ts";
 
 /**
@@ -43,16 +46,21 @@ export const questionTools = {
 
 export interface TeacherToolContext {
 	chatId: string;
+	/** The storage port plan tools persist through — the composition root
+	 *  (chat-handler) injects the wired adapter; the agent layer never
+	 *  imports one. */
+	chatStore: ChatStore;
 }
 
 /**
- * Per-request tool registry. Plan tools close over chatId for persistence;
- * question and job tools are context-free.
+ * Per-request tool registry. Plan tools close over the ctx (chatId + the
+ * injected ChatStore) for persistence; question and job tools are
+ * context-free.
  */
 export function createTeacherTools(ctx: TeacherToolContext): ToolSet {
 	return {
 		...questionTools,
 		...planTools(ctx),
-		...jobTools(ctx),
+		...jobTools(),
 	};
 }