fix: don't discard a finished task result when cancel races completion

[ryuhaneul]

Problem

1. Completion race loses finished results. TaskHub._run() obtains
   the CLI response, writes terminal status done, then awaits result
   delivery. A TaskHub.cancel() arriving in that window lands
   CancelledError at the delivery await: the handler overwrites the
   terminal status from done to cancelled and delivers only the
   TASKMEMORY excerpt — silently discarding the completed response the
   subprocess had already produced (and billed). From the user's side the
   task "finished into nothing".

2. Cancellations leave no trace. Neither TaskHub.cancel(),
   cancel_all(), nor the internal API cancel endpoint logged anything,
   so after the fact there is no way to tell who cancelled a task or when.

Fix

• Run the final delivery as an explicit task, keep a strong reference in
  a hub-level _pending_deliveries set (dropped via done-callback), and
  await it through asyncio.shield(). The CancelledError handler
  becomes a no-op once final delivery has started: the terminal done
  status stands and the finished result is delivered exactly once.
  Mid-run cancellation is unchanged (status cancelled, TASKMEMORY
  partial delivered). Shutdown drains any pending deliveries.
• Log the cancel paths: accepted / non-running in cancel(), a count in
  cancel_all(), and the internal API cancel including the requesting
  agent.

Semantics note: the cancel API's success=true means the request was
accepted; when completion wins the race the task's final status remains
done and the result is delivered.

Tests

Six scenarios: late cancel via cancel() preserves the result (delivered
once, status done, no partial cancelled delivery); same via
cancel_all(); mid-run cancel still delivers the TASKMEMORY partial with
status cancelled; the four cancel log flows (caplog); shutdown drains a
blocked pending delivery; API cancel logs the requesting agent.

4 files changed, +253/−17. Rollback = revert (no config/schema impact).

Note for merging alongside #164: touches tasks/hub.py in hunks
adjacent to #164 (per-session reasoning effort). Independent
single-commit PRs off current main; either merge order yields at most
trivial context conflicts.