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/task_deps.py

@@ -37,9 +37,12 @@ def is_task_ready(task_id: int) -> tuple[bool, list[str]]:
     """Return ``(ready, waiting_on)`` for a task.
 
     ``ready`` is True when the task has no declared dependency whose predecessor
-    is still un-done (``tasks.stage != 'done'``). ``waiting_on`` is the list of
-    predecessor work-item ids (e.g. ``["ORCH-010"]``) the task is still blocked
-    by — used for the Telegram waiting-line / Plane visibility.
+    is still un-done. ORCH-090 (adr-0026): the terminal set is
+    ``{done, cancelled}`` — a CANCELLED predecessor is terminal and no longer
+    blocks the dependent (the actual SQL predicate lives in
+    ``db.get_unfinished_dependencies`` / ``db.claim_next_job``). ``waiting_on`` is
+    the list of predecessor work-item ids (e.g. ``["ORCH-010"]``) the task is still
+    blocked by — used for the Telegram waiting-line / Plane visibility.
 
     never-raise: any error -> ``(True, [])`` (fail OPEN — consistent with the
     scheduler omitting the gate when the DB read fails; a transient error must