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/plane_sync.py

@@ -148,6 +148,13 @@ _PLANE_NAME_TO_KEY: dict[str, str] = {
     # this board status (enduro / API fallback) fail-closed — no UUID, no
     # confirm-deploy branch, no KeyError (accessed via .get).
     "Confirm Deploy": "confirm_deploy",
+    # ORCH-090: dedicated operator "STOP" status — the cancel trigger. Like
+    # ORCH-059's Confirm Deploy it is INTENTIONALLY ABSENT from _DEFAULT_STATES
+    # (fail-closed): environments without the status (enduro / API fallback)
+    # resolve `stop` to None via .get -> the cancel branch simply never activates
+    # (no UUID, no KeyError, no blind cancel). Create a STOP status with the
+    # `cancelled` group on the board to enable it (07-infra-requirements.md).
+    "STOP": "stop",
     # ORCH-066: meaningful per-stage / human-input statuses (layer B).
     "To Analyse":               "to_analyse",
     "Analysis":                 "analysis",