Pi-style ack-and-steer sub-agent protocol: dispatch mid-turn, steer via server wake turn

The job tools (research/demo/visual) switch from client-side turn-breaking
calls to pi's model: execute resolves IMMEDIATELY with a fire-and-forget
ack, the teacher's turn continues, and the result arrives later as an
injected user-role steer message that wakes the teacher in a new
server-side turn.

- contracts.ts: THE ASYNC PROTOCOL rewritten (ack now, steer later); steer
  types (SubagentSteerMessage, metadata.subagentResult + data-subagent-
  artifact part), pure buildSubagentSteer/steerDelivered/isSubagentFailure
- tools/jobs.ts: jobTools(ctx) factory — server-side execute creates the
  job (deduped by toolCallId), fires spawn with the dispatch-time input
  (the part isn't persisted yet at execute — a transcript scan would race)
- runner.ts: runJob(jobId, input?) builds the steer and hands it to the
  injected deliver seam; pipeline owns the done/error flip (a polling
  client re-syncs once, at the end); failures still steer
- server/jobs.ts (new pipeline): per-chat serialized queues on globalThis,
  idle gates (turn liveness + pending question, 1.5s grace), 409 retry
  with backoff, idempotent per jobId, stale-job recovery sweep (re-deliver
  or re-spawn); wake turns run through the same handleChatRequest
  composition via setTurnRunner injection
- chat-handler.ts: wires runner+deliver+tools ctx; drops spawnTurnJobs
  from onFinish; teacher default registry = question tools only
- plan.ts: PlanToolContext split out (plan tools need no job wiring)
- visual-maker: edit_svg added (pi parity — exact-match single
  replacement, fuzzy context on a miss); researcher: pi's kept/dropped
  source accounting restored
- prompts 20-jobs/40-demos: async discipline rewritten (announce and keep
  teaching, no waiting/polling/fabricating, parallel dispatch, teach with
  the artifact when it arrives)
- tests: jobs.test.ts rewritten (dispatch ack mid-turn, runner steers,
  wake pipeline: deferral/409/idempotency/failure), demos.test.tsx runner
  tests ported to runJob + fake deliver; 222 green

FLAGGED surgical touches outside the stream's owned files, for master
review at merge: src/core/contracts.ts (protocol doc + steer types),
src/core/ui-message.ts (job tool + data-part types), src/server/
chat-handler.ts (wiring), src/server/turn-service.ts (spawnTurnJobs
retirement), src/agent/teacher-agent.ts (default tools), src/agent/tools/
plan.ts (context type split).

Verified live on PORT=3003 with real models: teacher dispatched research
mid-turn, researcher ran real searches/fetches, steer injected on quiz
answer, wake turn wove the brief in citing its sources, job flipped done;
a client/wake race correctly 409'd.

src/server/turn-service.ts

@@ -6,7 +6,6 @@
  *
  *   buildTeacherContext — graph → instructions block (the W8 read channel)
  *   persistTurn        — the canonical state write (client + agent messages)
- *   spawnTurnJobs      — the async sub-agent protocol kick-off
  *   triggerExtraction  — the graph write channel: fires the extraction
  *                        pipeline (the sole graph writer) once a chat
  *                        crosses the v1 quiz-answer threshold. The trigger
@@ -22,7 +21,6 @@ import { buildGraphContext } from "../core/graph-context.ts";
 import { getGraph, pendingQuizAnswers } from "../core/graph.ts";
 import type { ChatStore, GraphStore } from "../core/ports.ts";
 import { EXTRACTION } from "../../config.ts";
-import type { JobRunner } from "../agent/subagents/runner.ts";
 import type { ExtractionPipeline } from "../agent/subagents/extraction.ts";
 
 /**
@@ -55,19 +53,6 @@ export function persistTurn(chatStore: ChatStore, chatId: string, messages: UIMe
 	}
 }
 
-/**
- * The async sub-agent protocol (contracts.ts): scan the last assistant
- * message for pending job tool parts and fire the background runners —
- * fire-and-forget, never awaited; the HTTP response is already gone.
- */
-export function spawnTurnJobs(jobRunner: JobRunner, chatId: string, messages: UIMessage[]): void {
-	try {
-		jobRunner.spawnPendingJobs(chatId, messages);
-	} catch (err) {
-		console.error(`[chat ${chatId}] job spawn failed:`, err);
-	}
-}
-
 /**
  * The graph write channel (vision §1 construction): the extraction pipeline
  * (extractor → auditor → merger — the sole graph writer) runs ASYNC once a