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/qg/checks.py

@@ -673,8 +673,19 @@ def check_branch_mergeable(repo: str, work_item_id: str, branch: str) -> tuple[b
             return False, reason
 
         try:
+            # ORCH-026 (Level A, A-2): proactive pre-merge rebase. When
+            # premerge_rebase_always is on, ALWAYS rebase onto the CURRENT
+            # origin/main under the held lease — even when branch_is_behind_main
+            # says "not behind". The ancestor check can miss a divergence
+            # (squash/force-push history, ORCH-073 phantom-merge class), so an
+            # unconditional rebase is a deterministic anti-phantom: it guarantees
+            # B carries A's code before merge. auto_rebase_onto_main is a cheap
+            # no-op on an already up-to-date branch (rc 0, push up-to-date, CI not
+            # retriggered). Kill-switch off -> 1:1 the ORCH-043 short-circuit
+            # below (rebase only when behind).
+            always = bool(getattr(settings, "premerge_rebase_always", False))
             # Double-check under the lease: another task may have just merged.
-            if not merge_gate.branch_is_behind_main(repo, branch):
+            if not always and not merge_gate.branch_is_behind_main(repo, branch):
                 logger.info("check_branch_mergeable: %s up-to-date with main", branch)
                 return True, "branch up-to-date with main"