feat: abort sub-agent jobs (real kill) + runner usage capture into JobRecord.usage

Wave 4 (agent-side observability + control), reconciled with the dock
contract (master relay 2026-09):

- abortJob in src/server/jobs.ts, exported through the server barrel for
  POST /api/activity/abort (master's thin route): fires the job's
  AbortController (real kill — the in-flight model call aborts, not just
  the next loop check), drops any queued result steer, flips the record
  to error "Aborted by learner" + activity "aborted" synchronously, and
  queues a one-line abort steer (buildAbortSteer) through the same wake
  pipeline so the teacher's promise doesn't hang in the lesson. Shares
  the result steer's deterministic id space, so abort + racing result
  can never both deliver (steerDelivered dedupe).
- Abort registry (src/agent/subagents/aborts.ts) on globalThis, mirroring
  the wake pipeline's module-state pattern; runner registers/unregisters
  per run, so the route's module instance sees the loop's controller.
- Runner usage capture: LoopContext (abortSignal + onUsage) threads into
  all three kind loops; per-step usage folds via accumulateJobUsage into
  JobRecord.usage (model from config MODELS, absent-never-zero), flushed
  in runJob's finally so an aborted job's money record survives the kill.
- Contracts: JobUsage token fields optional + lastStep (most recent
  call's accounting, for the dock's per-blob context line vs
  CONTEXT_LIMITS); SubagentResultMeta.aborted; AbortSteerMessage;
  buildAbortSteer; SteerMessage union generalized.
- demo-builder/visual-maker accept abortSignal + onUsage (visual folds
  generateText usage per iteration).
- stillRunning/finishJob/jobContext guards: an aborted job is never
  overwritten by the runner (terminal no matter what the wake does);
  failJob and the pump's finalizeJob keep the flip.
- UI: abort steer renders a quiet aborted chip (job-aborted-badge).
- Prompt 20-jobs.md: one line about aborted jobs.

Tests: accumulateJobUsage folds (undefined-preservation, lastStep),
runner usage capture, real-kill abort (sync flip, silent unwind,
abort steer wakes), usage-so-far survives the kill, aborting a
delivering job drops the queued result steer, outcome reasons,
abort-chip render. 330/330, typecheck/lint/build green.

src/agent/subagents/demo-builder.ts

@@ -1,7 +1,7 @@
 import { ToolLoopAgent, stepCountIs, tool, type LanguageModel, type ToolSet } from "ai";
 import { z } from "zod";
 import { modelFor } from "../models.ts";
-import type { DemoResult } from "../../core/contracts.ts";
+import type { DemoResult, UsageStepLike } from "../../core/contracts.ts";
 import { getHeadless, type DemoRunResult, type HeadlessApi } from "./headless.ts";
 
 /**
@@ -32,6 +32,10 @@ export interface DemoBuilderOptions {
 	maxSteps?: number;
 	/** Live activity-line callback (updates the job record). */
 	onActivity?: (line: string) => void;
+	/** Kills the run mid-model-call (2026-09 real-kill abort semantics). */
+	abortSignal?: AbortSignal;
+	/** Money channel: one call per agent step, folded into JobRecord.usage. */
+	onUsage?: (usage: UsageStepLike) => void;
 }
 
 const INSTRUCTIONS = `You are a demo builder — a specialist sub-agent inside a learning application. You
@@ -189,6 +193,8 @@ export async function runDemoBuilder(input: DemoBuilderInput, opts: DemoBuilderO
 	try {
 		const result = await agent.generate({
 			prompt,
+			abortSignal: opts.abortSignal,
+			onStepFinish: ({ usage }) => opts.onUsage?.(usage),
 			onToolExecutionStart: ({ toolCall }) => {
 				opts.onActivity?.(toolCall.toolName);
 			},