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/jobs.ts

@@ -1,6 +1,5 @@
 import { tool, type ToolSet } from "ai";
 import { z } from "zod";
-import type { TeacherToolContext } from "../tools.ts";
 
 /**
  * Async sub-agent job tools (W2: research; W6 adds demo + visual).
@@ -72,10 +71,10 @@ export const visualTool = tool({
 
 /**
  * Per-request job tool registry. Context-free (the runner resolves
- * everything from the chat record by toolCallId) — the ctx parameter keeps
- * the createTeacherTools factory shape stable.
+ * everything from the chat record by toolCallId). Takes no dependencies —
+ * part of the dependency-free default registry in teacher-agent.ts.
  */
-export function jobTools(ctx: TeacherToolContext): ToolSet {
+export function jobTools(): ToolSet {
 	return {
 		research: researchTool,
 		demo: demoTool,