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.

README.md

@@ -58,7 +58,7 @@ graph TB
     end
 
     subgraph TEACHER["Teacher agent (src/agent)"]
-        CH["chat-handler.ts (framework-free)<br/>graph context · persist · spawn jobs · emit"]
+        CH["chat-handler.ts — turn composition root<br/>(services: context · persist · spawn · fold)"]
         TA["ToolLoopAgent<br/>system-prompt.md + prompts/*.md fragments<br/>+ graph-context block"]
         QT["quiz · ask_user_question<br/>(client-side tools, no execute)"]
         PT["plan_* tools (server-side,<br/>mutate the chat's tree)"]
@@ -125,7 +125,9 @@ src/
     tools/jobs.ts               research / demo / visual tool surfaces
     subagents/                  researcher · demo-builder · visual-maker,
                                 runner (async protocol), headless Chrome helper
-  server/chat-handler.ts       one chat turn: stream, persist, spawn, emit
+  server/chat-handler.ts       one chat turn: compose agent + services, stream
+  server/turn-service.ts       turn services: teacher context · persist ·
+                                spawn jobs · fold-into-graph (error-contained)
   server/stores.ts              THE composition root — the one place adapters wire in
   core/contracts.ts             THE contracts file — types + protocol docs
   core/ports.ts                 storage ports — ChatStore · GraphStore · JobStore