feat(gateway): propagate sessions to subtasks + auto-emit milestone progress

Task #156 (sessions): pre-gateway flow created a session for the whole
task tree at once, so subtasks were visible in the PM's group chat the
moment they existed. The gateway creates subtasks one-by-one via
delegate(), losing that wiring. Added
MessagingService.propagate_sessions_to_subtask and threaded it through
the choreographer's _create_subtask_from_inputs. ChoreographerDeps grew
an optional `messaging` field so existing test wirings keep working.

Task #155 (progress): smoke-9 had zero progress entries because the dev
never called progress() explicitly. Added _record_milestone_progress and
fire it server-side from two natural milestones — open_pr ("opened PR
#N", 70%) and i_am_done ("submitted for QA review", 90%). Best-effort
write (contextlib.suppress) so a progress failure cannot break the verb
path. Extracted _open_pr_success_envelope to keep cyclomatic rank ≤ B.

tests/unit/services/test_propagate_sessions_to_subtask.py

@@ -0,0 +1,98 @@
+"""Task #156: MessagingService.propagate_sessions_to_subtask.
+
+Tests the helper's call shape and idempotency contract without going
+through the DB. Integration coverage (real DB, real linking) lives in
+``tests/integration/test_messaging_service.py``.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
+
+import pytest
+from roboco.models.session import SessionTaskRelationshipType
+from roboco.services.messaging import MessagingService
+
+
+def _link(session_id: object, relationship_type: str) -> MagicMock:
+    link = MagicMock()
+    link.session_id = session_id
+    link.relationship_type = relationship_type
+    return link
+
+
+@pytest.mark.asyncio
+async def test_propagate_links_every_parent_session_to_subtask() -> None:
+    """Every link on the parent gets re-attached to the new subtask."""
+    svc = MessagingService.__new__(MessagingService)
+    parent_session = uuid4()
+    review_session = uuid4()
+    svc.get_sessions_for_task = AsyncMock(  # type: ignore[method-assign]
+        return_value=[
+            _link(parent_session, "discussion"),
+            _link(review_session, "review"),
+        ]
+    )
+
+    calls: list[dict[str, Any]] = []
+
+    async def fake_link(**kwargs: Any) -> Any:
+        calls.append(kwargs)
+        link = MagicMock()
+        link.session_id = kwargs["session_id"]
+        link.task_id = kwargs["task_id"]
+        return link
+
+    svc.link_session_to_task = fake_link  # type: ignore[method-assign]
+
+    parent_id = uuid4()
+    subtask_id = uuid4()
+    added_by = uuid4()
+    expected_sessions = {parent_session, review_session}
+    out = await svc.propagate_sessions_to_subtask(parent_id, subtask_id, added_by)
+    assert len(out) == len(expected_sessions)
+    assert {c["session_id"] for c in calls} == expected_sessions
+    # Every propagated link must be non-primary — primary is the subtask's
+    # own slot, never inherited from the parent.
+    assert all(c["is_primary"] is False for c in calls)
+    # Every call must target the new subtask (not the parent).
+    assert {c["task_id"] for c in calls} == {subtask_id}
+    # Relationship types preserved.
+    rels = {c["relationship_type"] for c in calls}
+    assert SessionTaskRelationshipType.DISCUSSION in rels
+    assert SessionTaskRelationshipType.REVIEW in rels
+
+
+@pytest.mark.asyncio
+async def test_propagate_no_parent_sessions_returns_empty() -> None:
+    """When the parent has no session links, propagation is a no-op."""
+    svc = MessagingService.__new__(MessagingService)
+    svc.get_sessions_for_task = AsyncMock(return_value=[])  # type: ignore[method-assign]
+    svc.link_session_to_task = AsyncMock()  # type: ignore[method-assign]
+
+    out = await svc.propagate_sessions_to_subtask(uuid4(), uuid4(), uuid4())
+    assert out == []
+    svc.link_session_to_task.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_propagate_unknown_relationship_type_defaults_to_discussion() -> None:
+    """Garbage relationship_type on the parent link doesn't crash; it
+    defaults to DISCUSSION so the subtask is still linked."""
+    svc = MessagingService.__new__(MessagingService)
+    svc.get_sessions_for_task = AsyncMock(  # type: ignore[method-assign]
+        return_value=[_link(uuid4(), "definitely-not-a-real-type")]
+    )
+    calls: list[dict[str, Any]] = []
+
+    async def fake_link(**kwargs: Any) -> Any:
+        calls.append(kwargs)
+        return MagicMock()
+
+    svc.link_session_to_task = fake_link  # type: ignore[method-assign]
+
+    await svc.propagate_sessions_to_subtask(uuid4(), uuid4(), uuid4())
+    assert len(calls) == 1
+    assert calls[0]["relationship_type"] == SessionTaskRelationshipType.DISCUSSION