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/agent/teacher-agent.ts

@@ -2,8 +2,8 @@ import * as fs from "node:fs";
 import * as path from "node:path";
 import { ToolLoopAgent, stepCountIs, type LanguageModel, type ToolSet } from "ai";
 import { PATHS } from "../../config.ts";
-import { createTeacherTools, jobTools, questionTools } from "./tools.ts";
 import { modelFor } from "./models.ts";
+import { questionTools } from "./tools.ts";
 
 /**
  * The teacher agent. Everything pi used to provide (agent loop, tool calling,
@@ -56,13 +56,15 @@ export function createTeacherAgent(model?: LanguageModel, tools?: ToolSet, graph
 		// Reasoning tokens stream as reasoning parts into the UI's collapsible block.
 		model: model ?? modelFor("teacher"),
 		instructions: withGraphContext(graphContext),
-		// Tools are per-request (plan tools close over the injected ctx);
-		// chat-handler passes createTeacherTools({ chatId, chatStore }). The
+		// Tools are per-request (plan tools close over the injected ctx; job
+		// tools close over the job store + spawn seam) — chat-handler passes
+		// createTeacherTools({ chatId, chatStore, jobStore, spawn }). The
 		// dependency-free default is the question-only set (client-side tools,
 		// no storage needed) — used when tests pass a bare mock model.
-		tools: tools ?? { ...questionTools, ...jobTools() },
-		// Question and job tools are client-side — a call ends the server turn
-		// automatically. stepCountIs is just the safety net.
+		tools: tools ?? { ...questionTools },
+		// Question tools are client-side (a call ends the server turn);
+		// job tools resolve immediately (fire-and-forget ack). stepCountIs is
+		// just the safety net.
 		stopWhen: stepCountIs(20),
 	});
 }