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.

tests/demos.test.tsx

@@ -49,7 +49,7 @@ import { runDemoBuilder } from "../src/agent/subagents/demo-builder.ts";
 import { headlessOverride, type DemoRunResult, type HeadlessApi } from "../src/agent/subagents/headless.ts";
 import { createJobRunner, demoModelOverride, visualModelOverride } from "../src/agent/subagents/runner.ts";
 import { runVisualMaker } from "../src/agent/subagents/visual-maker.ts";
-import { buildSubagentSteer, isSubagentFailure, type SubagentSteerMessage } from "../src/core/contracts.ts";
+import { buildAbortSteer, buildSubagentSteer, isSubagentFailure, type SubagentSteerMessage } from "../src/core/contracts.ts";
 import type { JobRecord } from "../src/core/contracts.ts";
 import { createTeacherTools } from "../src/agent/tools.ts";
 import { chatStore } from "../src/store/chats.ts";
@@ -342,6 +342,18 @@ describe("steer card rendering (the delivered artifact)", () => {
 		assert.ok(!html.includes("Looking at the render"), "model-facing notes never render");
 	});
 
+	it("renders the abort chip for an aborted job's steer (2026-09 dock semantics)", () => {
+		const steer = buildAbortSteer({ id: "jc", kind: "research", toolCallId: "cc" });
+		assert.equal(steer.metadata.subagentResult.aborted, true);
+		assert.equal(steer.parts.length, 1, "no artifact part on an abort");
+		const html = renderToStaticMarkup(<SubagentSteerCard message={steer as never} />);
+		assert.match(html, /job-aborted-badge/);
+		assert.match(html, /research/, "the job's kind, lower-cased like the complete-badge idiom");
+		assert.match(html, /aborted by the learner/);
+		// the teacher-facing protocol text never renders for the learner
+		assert.ok(!html.includes("no brief is coming"), "no protocol glue in the chip");
+	});
+
 	it("renders a quiet Research-complete one-liner whose collapsed <details> carries the full brief", () => {
 		const steer = buildSubagentSteer(
 			{ id: "jr", kind: "research", toolCallId: "cr" },