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>

tests/test_serial_gate.py

@@ -180,9 +180,11 @@ def test_snapshot_shape_and_never_raises(monkeypatch):
 def test_registries_unchanged():
     from src.stages import STAGE_TRANSITIONS
     from src.qg.checks import QG_CHECKS
+    # ORCH-090 (adr-0026): `cancelled` is added as a terminal SINK (parallel to
+    # `done`), NOT a new pipeline edge — serial-gate FIFO semantics are unchanged.
     assert set(STAGE_TRANSITIONS) == {
         "created", "analysis", "architecture", "development", "review",
-        "testing", "deploy-staging", "deploy", "done",
+        "testing", "deploy-staging", "deploy", "done", "cancelled",
     }
     # No serial-gate QG check was introduced (the gate is a scheduler condition).
     assert not any("serial" in k for k in QG_CHECKS), "no new QG check expected"