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/merge_gate.py

@@ -340,6 +340,21 @@ def release_merge_lease(repo: str, branch: str | None = None) -> None:
         logger.warning("merge-lease release error for %s: %s", repo, e)
 
 
+def current_lease_holder(repo: str) -> str | None:
+    """ORCH-090: branch currently holding the per-repo merge-lease, or None.
+
+    Read-only helper used by ``cancel.in_critical_window`` to decide whether a STOP
+    must be DEFERRED (the task is mid-merge). Never raises -> None on missing/corrupt
+    lease or any error (the caller treats an error as fail-CLOSED itself).
+    """
+    try:
+        existing = _read_lease(_lease_path(repo))
+        return existing.get("branch") if existing else None
+    except Exception as e:  # noqa: BLE001 - never-raise
+        logger.warning("current_lease_holder error for %s: %s", repo, e)
+        return None
+
+
 # ---------------------------------------------------------------------------
 # ORCH-065: proactive stale/dead merge-lease reclaim (Problem B)
 # ---------------------------------------------------------------------------