feat(ORCH-026): task dependencies (B waits for A) + single-repo merge serialization

Level A — merge/deploy serialization within one repo: reuse the existing
ORCH-043/065 merge-lease (no new mechanism); the only new logic is an
unconditional pre-merge rebase in check_branch_mergeable — under the held
lease, auto_rebase_onto_main is ALWAYS called when premerge_rebase_always
(default True), not just when the branch is behind. No-op on an up-to-date
branch (rebase keeps HEAD, force-with-lease -> "Everything up-to-date", CI
not triggered). Kill-switch off -> ORCH-043 behaviour 1:1.

Level B — declarative task dependencies: additive job_deps table
(CREATE ... IF NOT EXISTS, no live-DB migration); claim_next_job gate
(NOT EXISTS) defers a job whose depends-on tasks are not yet 'done' without
occupying a max_concurrency slot; inert on empty job_deps -> zero regression.
New leaf src/task_deps.py (never-raise): is_task_ready (fail-open), DFS cycle
detection + Blocked/alert, declare/ingest_plane_relations (db source never
hits the network on the hot path), snapshot. Telegram waiting-line, /queue
observability, reconciler skip + cycle backstop, reaper untouched.

Invariants unchanged: STAGE_TRANSITIONS, QG_CHECKS registry (dep gate is a
claim_next_job врезка, not a registered QG), DB schema of existing tables,
HTTP endpoints; non-self repos remain a no-op on empty deps/scope.

Flags: ORCH_PREMERGE_REBASE_ALWAYS, ORCH_TASK_DEPS_ENABLED, ORCH_TASK_DEPS_SOURCE.
Docs: docs/architecture/README.md, CLAUDE.md, .env.example, CHANGELOG.md,
adr-0015. Tests: tests/test_orch026_*.py (64 tests); full suite 991 green.

Refs: ORCH-026

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/webhooks/plane.py

@@ -608,6 +608,17 @@ async def start_pipeline(data: dict, project_id: str = ""):
     except Exception as e:
         logger.error(f"Failed to launch analyst for {work_item_id}: {e}")
 
+    # ORCH-026 (B-1): import declared Plane `blocked-by` relations into job_deps
+    # (only for task_deps_source = plane|hybrid; default `db` -> no-op, no Plane
+    # call). Best-effort, never-raise: a Plane outage must not block the start.
+    try:
+        from .. import task_deps
+        n = task_deps.ingest_plane_relations(task_id, plane_id, plane_project_id)
+        if n:
+            logger.info(f"Task {task_id}: ingested {n} blocked-by dependency edge(s)")
+    except Exception as e:
+        logger.warning(f"Task {task_id}: dependency ingestion skipped: {e}")
+
 
 async def handle_comment(data: dict, project_id: str = ""):
     """Status-only verdict model: comments NEVER drive the pipeline.