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.

tests/jobs.test.ts

@@ -26,10 +26,13 @@ import { createTeacherAgent } from "../src/agent/teacher-agent.ts";
 import { handleChatRequest } from "../src/server/chat-handler.ts";
 import { chatStore } from "../src/store/chats.ts";
 import { jobStore } from "../src/store/jobs.ts";
-import { researchModelOverride, runResearchJob, spawnPendingJobs } from "../src/agent/subagents/runner.ts";
+import { createJobRunner, researchModelOverride } from "../src/agent/subagents/runner.ts";
 import { createResearcherAgent } from "../src/agent/subagents/researcher.ts";
 import { PATHS } from "../config.ts";
 
+/** The runner with the same wired fs ports the chat-handler injects. */
+const jobRunner = createJobRunner({ chatStore, jobStore });
+
 const JOBS_DIR = path.join(PATHS.data, "jobs");
 
 const createdChats: string[] = [];
@@ -176,7 +179,7 @@ describe("spawn hook (async protocol §2)", () => {
 		trackJob(job!.id);
 
 		// Re-running the spawn hook (idempotency) does not create a second job.
-		spawnPendingJobs(chat.id, saved.messages);
+		jobRunner.spawnPendingJobs(chat.id, saved.messages);
 		assert.equal(jobStore.listJobs(chat.id).length, 1, "no duplicate job for the same tool part");
 	});
 });
@@ -270,7 +273,7 @@ describe("research runner (async protocol §3)", () => {
 			const job = jobStore.createJob({ kind: "research", chatId: chat.id, toolCallId: "call-run1" });
 			trackJob(job.id);
 
-			await runResearchJob(job.id, { model: researcherModel() });
+			await jobRunner.runResearchJob(job.id, { model: researcherModel() });
 
 			// The job is done, with the activity line from the tool call.
 			const done = jobStore.getJob(job.id)!;
@@ -313,7 +316,7 @@ describe("research runner (async protocol §3)", () => {
 		const job = jobStore.createJob({ kind: "research", chatId: chat.id, toolCallId: "call-err1" });
 		trackJob(job.id);
 
-		await runResearchJob(job.id, { model: failing });
+		await jobRunner.runResearchJob(job.id, { model: failing });
 
 		const errored = jobStore.getJob(job.id)!;
 		assert.equal(errored.status, "error");