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/plan-tree.test.ts

@@ -236,7 +236,7 @@ function newChat(title: string): string {
 describe("plan tools (execute against the real store)", () => {
 	it("plan_init creates and persists the tree; result is ok with the full tree", async () => {
 		const chatId = newChat("Plan tools: init");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 
 		const res = await exec(tools, "plan_init", {
 			nodes: [
@@ -258,7 +258,7 @@ describe("plan tools (execute against the real store)", () => {
 
 	it("plan_init refuses to re-init and never silently wipes the existing tree", async () => {
 		const chatId = newChat("Plan tools: re-init");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 		await exec(tools, "plan_init", { nodes: [{ id: "a", title: "A" }] });
 
 		const res = await exec(tools, "plan_init", {
@@ -274,7 +274,7 @@ describe("plan tools (execute against the real store)", () => {
 
 	it("plan_init returns an error result on invalid input (cycle) without persisting", async () => {
 		const chatId = newChat("Plan tools: bad init");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 		const res = await exec(tools, "plan_init", {
 			nodes: [
 				{ id: "a", title: "A" },
@@ -293,7 +293,7 @@ describe("plan tools (execute against the real store)", () => {
 
 	it("mutation round-trip: add node with parents, set current, mark done — each persists", async () => {
 		const chatId = newChat("Plan tools: round-trip");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 
 		await exec(tools, "plan_init", {
 			nodes: [
@@ -331,7 +331,7 @@ describe("plan tools (execute against the real store)", () => {
 
 	it("invalid mutations return ok:false with the current tree and persist nothing", async () => {
 		const chatId = newChat("Plan tools: invalid ops");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 		await exec(tools, "plan_init", { nodes: [{ id: "a", title: "A" }] });
 		const before = chatStore.getChat(chatId)!.tree!;
 
@@ -353,7 +353,7 @@ describe("plan tools (execute against the real store)", () => {
 
 	it("mutation tools work on a fresh (uninitialized) chat: the tree seeds from empty", async () => {
 		const chatId = newChat("Plan tools: no init");
-		const tools = planTools({ chatId });
+		const tools = planTools({ chatId, chatStore });
 		const res = await exec(tools, "plan_add_node", { id: "a", title: "A" });
 		assert.equal(res.ok, true);
 		assert.equal(res.tree.nodes.length, 1);
@@ -361,7 +361,7 @@ describe("plan tools (execute against the real store)", () => {
 	});
 
 	it("unknown chat returns ok:false with the empty tree", async () => {
-		const tools = planTools({ chatId: "chat_nonexistent" });
+		const tools = planTools({ chatId: "chat_nonexistent", chatStore });
 		const res = await exec(tools, "plan_mark_done", { id: "a" });
 		assert.equal(res.ok, false);
 		assert.match(res.error!, /Chat not found/);