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>

tests/test_orch026_queue_observability.py

@@ -0,0 +1,90 @@
+"""ORCH-026 — /queue task_deps observability (TC-G02, G-2).
+
+task_deps.snapshot() is a read-only summary (NOT a source of truth) exposing the
+declared edges, blocked tasks and any detected cycle. It must never raise.
+"""
+import os
+import tempfile
+
+import pytest
+
+os.environ["ORCH_DB_PATH"] = os.path.join(tempfile.gettempdir(), "test_orch026_queue_obs.db")
+os.environ.setdefault("ORCH_GITEA_TOKEN", "test-token")
+os.environ.setdefault("ORCH_PLANE_API_TOKEN", "test-token")
+
+import src.db as db  # noqa: E402
+from src.db import init_db, get_db  # noqa: E402
+from src import task_deps  # noqa: E402
+
+
+@pytest.fixture(autouse=True)
+def fresh_db(tmp_path, monkeypatch):
+    dbfile = tmp_path / "obs.db"
+    monkeypatch.setattr(db.settings, "db_path", str(dbfile))
+    monkeypatch.setattr(db.settings, "task_deps_enabled", True, raising=False)
+    monkeypatch.setattr(db.settings, "task_deps_source", "db", raising=False)
+    init_db()
+    yield
+
+
+def _make_task(work_item_id, stage="development"):
+    conn = get_db()
+    cur = conn.execute(
+        "INSERT INTO tasks (plane_id, work_item_id, repo, branch, stage) "
+        "VALUES (?, ?, ?, ?, ?)",
+        (work_item_id, work_item_id, "orchestrator", f"feature/{work_item_id}", stage),
+    )
+    tid = cur.lastrowid
+    conn.commit()
+    conn.close()
+    return tid
+
+
+def test_snapshot_shape_empty():
+    snap = task_deps.snapshot()
+    assert snap["enabled"] is True
+    assert snap["source"] == "db"
+    assert snap["edges"] == 0
+    assert snap["blocked_tasks"] == []
+    assert snap["cycle"] is None
+
+
+def test_snapshot_reports_blocked_task():
+    a = _make_task("ORCH-100", stage="development")
+    b = _make_task("ORCH-101", stage="development")
+    db.add_dependency(b, a)
+    snap = task_deps.snapshot()
+    assert snap["edges"] == 1
+    assert len(snap["blocked_tasks"]) == 1
+    bt = snap["blocked_tasks"][0]
+    assert bt["work_item_id"] == "ORCH-101"
+    assert "ORCH-100" in bt["waiting_on"]
+    assert snap["cycle"] is None
+
+
+def test_snapshot_reports_cycle():
+    a = _make_task("ORCH-102")
+    b = _make_task("ORCH-103")
+    db.add_dependency(a, b)
+    db.add_dependency(b, a)
+    snap = task_deps.snapshot()
+    assert snap["cycle"] is not None
+    assert "ORCH-102" in snap["cycle"] or "ORCH-103" in snap["cycle"]
+
+
+def test_snapshot_never_raises(monkeypatch):
+    monkeypatch.setattr(db, "get_dependency_edges",
+                        lambda: (_ for _ in ()).throw(RuntimeError("db down")),
+                        raising=False)
+    snap = task_deps.snapshot()
+    assert snap["edges"] == 0
+    assert snap["blocked_tasks"] == []
+
+
+def test_queue_endpoint_includes_task_deps(monkeypatch):
+    """GET /queue payload carries the task_deps block (read-only)."""
+    import asyncio
+    from src import main
+    payload = asyncio.run(main.queue())
+    assert "task_deps" in payload
+    assert "enabled" in payload["task_deps"]