feat(cancel): STOP-status task cancellation + relaunch-hole close (ORCH-090)

Introduce the dedicated Plane STOP status as a single declarative task-cancel
mechanism: stop the active agent (graceful SIGTERM cascade), cancel all jobs
(terminal `cancelled`, never requeued), remove the worktree + delete the remote
feature branch (never main, never force-push), drive the task to the new
system-terminal state `cancelled` and tombstone the natural keys so a later
"To Analyse" re-creates it from scratch (docs artefacts preserved). STOP during a
critical merge/deploy window is deferred until the irreversible step finishes
honestly. Also closes the relaunch hole: handle_status_start relaunch is gated to
the `analysis` stage; the only pipeline-start entry point remains "To Analyse".

Cross-cutting (adr-0026): the "task terminal" predicate is widened {done} ->
{done, cancelled} in serial_gate / task_deps / stages sink + reaper/worker
requeue guards. STAGE_TRANSITIONS exit-gates / QG_CHECKS / check_* are unchanged
(`cancelled` is a sink, not a new edge). Additive, never-raise, restart-safe,
under kill-switch ORCH_STOP_STATUS_ENABLED (off -> zero regression).

New: src/cancel.py (leaf), src/gitea.py (delete_remote_branch), tasks columns
cancelled_at/cancel_requested_at, jobs status `cancelled`, GET /queue `stop` block.
Tests: tests/test_stop_status.py (TC-01..TC-14 + D7); full suite green (1345).
Docs updated in-PR (architecture README, CLAUDE.md, README.md, .env.example,
CHANGELOG). ADR-001 D4 refinement: plane_issue_id is tombstoned too (the lookup
ORs on it) — original UUID recoverable from the parseable suffix.

Refs: ORCH-090

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/queue_worker.py

@@ -187,12 +187,18 @@ class QueueWorker:
                 # launch error so the job does not wedge as 'running' forever.
                 logger.error(f"Worker failed to launch job {job['id']}: {e}")
                 try:
-                    from .db import get_job, mark_job
+                    from .db import get_job, mark_job, get_task
 
                     j = get_job(job["id"])
                     attempts = j.get("attempts", 0) if j else 0
                     max_attempts = j.get("max_attempts", 2) if j else 2
-                    if attempts < max_attempts:
+                    # ORCH-090 (adr-0026 / TR-2): never requeue a job whose task is
+                    # already terminal ({done,cancelled}) — a STOP that landed between
+                    # claim and launch must win over the retry budget.
+                    task = get_task(job.get("task_id")) if job.get("task_id") else None
+                    if task and task.get("stage") in ("done", "cancelled"):
+                        mark_job(job["id"], "cancelled", error=f"launch error (task terminal): {e}")
+                    elif attempts < max_attempts:
                         mark_job(job["id"], "queued", error=f"launch error: {e}")
                     else:
                         mark_job(job["id"], "failed", error=f"launch error: {e}")