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.

components/artifacts/steer-card.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { CheckIcon } from "lucide-react";
+import { CheckIcon, CircleX } from "lucide-react";
 import type { ResearchResult, SubagentArtifactData, TeacherUIMessage } from "@/src/core";
 import { isSubagentFailure } from "@/src/core";
 import { DemoOutputCard } from "@/components/artifacts/demo-card";
@@ -27,9 +27,15 @@ import { VisualOutputCard } from "@/components/artifacts/visual-card";
 export function SubagentSteerCard({ message }: { message: TeacherUIMessage }) {
 	const artifactPart = message.parts.find((p) => p.type === "data-subagent-artifact");
 	const data = artifactPart?.data as SubagentArtifactData | undefined;
+	// The abort steer (2026-09 dock semantics): no artifact part, only the
+	// metadata marker — a quiet chip, mirroring the complete-badge idiom.
 	if (!data) {
+		const meta = (
+				message.metadata as { subagentResult?: { aborted?: boolean; kind?: string } } | undefined
+			)?.subagentResult;
+		if (meta?.aborted) return <AbortedBadge kind={meta.kind ?? "job"} />;
 		return (
-			<div className="rounded-xl border bg-card p-4 text-sm text-muted-foreground">
+				<div className="rounded-xl border bg-card p-4 text-sm text-muted-foreground">
 				A sub-agent finished, but its result could not be displayed.
 			</div>
 		);
@@ -48,6 +54,19 @@ export function SubagentSteerCard({ message }: { message: TeacherUIMessage }) {
 	}
 }
 
+/** The aborted-job chip: mirrors the complete-badge one-liner — the
+ * teacher's wake turn acknowledges the abort in the lesson itself,
+ * this just marks what happened. */
+function AbortedBadge({ kind }: { kind: string }) {
+	return (
+		<div data-testid="job-aborted-badge" className="flex items-center gap-2 text-xs text-muted-foreground">
+			<CircleX className="size-3.5" />
+			<span className="font-medium uppercase tracking-wide">{kind}</span>
+			<span>aborted by the learner</span>
+		</div>
+	);
+}
+
 /** Quiet one-liner, the dispatch-settled idiom — the lesson lives in the
  * teacher's wake turn; the badge just marks delivery. The persisted brief
  * rides along in a collapsed <details> (the wave-1 "view full research"