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/main.py

@@ -171,6 +171,7 @@ async def queue():
     from . import task_deps
     from . import serial_gate
     from . import labels
+    from . import cancel
     from .disk_watchdog import disk_watchdog
     from .build_cache_pruner import build_cache_pruner
     return {
@@ -191,6 +192,10 @@ async def queue():
         # ORCH-089 (D7): auto-mode-by-label observability (read-only) — kill-switch,
         # label names, scope. Additive block.
         "auto_labels": labels.snapshot(),
+        # ORCH-090 (AC-10): STOP-cancellation observability (read-only) — kill-switch,
+        # repo scope, cancelled/deferred counts, recent cancellations. Additive block;
+        # never-raise.
+        "stop": cancel.snapshot(),
         # ORCH-063 (FR-6 / AC-7): disk-watchdog observability (read-only) —
         # enabled, threshold, interval, last measurement per host-path. Additive
         # block; never-raise (status() returns {"enabled": ...} minimum on error).