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/main.py

@@ -148,6 +148,7 @@ async def queue():
     from .job_reaper import reaper
     from . import post_deploy
     from . import merge_gate
+    from . import task_deps
     return {
         "counts": job_status_counts(),
         "max_concurrency": worker.max_concurrency,
@@ -157,5 +158,8 @@ async def queue():
         "reaper": reaper.status(),
         "post_deploy": post_deploy.status(),
         "merge_verify": merge_gate.merge_verify_status(),
+        # ORCH-026 (G-2): declarative task-dependency observability (read-only,
+        # NOT a source of truth) — declared edges, blocked tasks, detected cycle.
+        "task_deps": task_deps.snapshot(),
         "recent": recent_jobs(10),
     }