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/prompts/20-jobs.md

@@ -27,3 +27,7 @@ Dispatch as many research jobs as you genuinely need — in one turn or across t
 parallel when the questions are independent — but don't fan out on one question; the
 researcher searches multiple angles on its own. If research fails, you'll get a failure
 message: say so plainly and continue with what you can establish, flagging the uncertainty.
+
+A job can also be **aborted by the learner** — you'll get a one-line "was aborted"
+message instead of the result. Acknowledge it in one sentence and continue teaching
+without it (offering to re-dispatch is fine if the learner wants the result).