Handle ResumeResponse.is_done from the K-parallel backend

[yuxiang-wu]

This branch contains the full parallelization backend work: the foundational correctness commit (atomic step allocation / task attribution / completion CAS) and the K-parallel feature commit described below. Deploy order: supabase migration -> meta-agent -> CLI patch.

fix: handle ResumeResponse.is_done from PR-2 backend (patch release)

The PR-2 backend's /runs/{id}/resume can finalize a run itself (promote a
scored-but-interrupted node → step budget met → run flipped completed, no runnable
work), signalling it with a new additive is_done: bool on ResumeResponse. This
backend deploys BEFORE the CLI's parallel features, so today's CLI must handle it.

Bug (pre-fix): is_done was ignored. Manual weco resume entered the
task-polling loop, saw run.status == "completed", and returned
OptimizationResult(success=False, reason="user_requested_stop") — reporting a
completed run as stopped/failed (weco/optimizer.py:307-317, entered at :998).
The silent auto-resume path (_silent_resume returning a bare bool) re-entered the
loop and hit the same misreport.

Fix (weco/optimizer.py):

• is_done is parsed defensively — absent → False, so the CLI stays correct
  against an older backend (unchanged legacy behavior).
• Manual resume_optimization short-circuits on is_done: prints a "Run already
  complete" success message (matching the existing completion output), reuses
  offer_apply_best_solution to fetch/apply the winning code, and exits True
  without entering the task loop.
• _silent_resume now returns _SilentResumeOutcome{RESUMED, ALREADY_COMPLETE, FAILED}; _run_loop_with_auto_resume treats ALREADY_COMPLETE as terminal
  success (fires on_complete, returns a completed result, stops retrying) —
  mirroring how a normal is_done exits the loop.

Tests: tests/test_resume_is_done.py (is_done true → no loop / success; is_done
absent → legacy loop path; is_done false → legacy loop path) and two new cases in
tests/test_auto_resume.py for the ALREADY_COMPLETE outcome. Fully mocked, no
network. Run: pytest tests/test_auto_resume.py tests/test_resume_is_done.py.

Ships last in the deploy order (supabase → meta-agent → CLI).

[yuxiang-wu]

Closing for now — this branch will be part of a single PR covering the complete parallelization feature once all phases are implemented.