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/gateway/test_delegate_propagates_sessions.py

@@ -0,0 +1,138 @@
+"""Task #156: delegate() must thread parent sessions onto the new subtask.
+
+Pre-gateway flow created sessions for whole task trees at once, so subtasks
+were visible in the group chat the PM was already using. The gateway's
+delegate() creates subtasks one-by-one — without this step the new agent
+spawns into an empty channel and can't see the PM's prior discussion.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+from typing import Any
+from unittest.mock import AsyncMock, MagicMock
+from uuid import uuid4
+
+import pytest
+from roboco.services.gateway.choreographer import Choreographer, ChoreographerDeps
+from roboco.services.gateway.choreographer._impl import DelegateInputs
+
+
+def _make_deps(**overrides: Any) -> ChoreographerDeps:
+    base: dict[str, Any] = {
+        "task": AsyncMock(),
+        "work_session": AsyncMock(),
+        "git": AsyncMock(),
+        "a2a": AsyncMock(),
+        "journal": AsyncMock(),
+        "audit": AsyncMock(),
+        "evidence_repo": AsyncMock(),
+        "messaging": AsyncMock(),
+    }
+    base.update(overrides)
+    task = base["task"]
+    task.session = MagicMock()
+    task.session.begin_nested = MagicMock(
+        return_value=MagicMock(
+            __aenter__=AsyncMock(return_value=None),
+            __aexit__=AsyncMock(return_value=False),
+        )
+    )
+    repo = base["evidence_repo"]
+    for method in (
+        "list_unread_a2a",
+        "list_unread_mentions",
+        "list_pending_notifications",
+        "task_metadata_gaps",
+        "recent_team_activity",
+        "blockers_in_lane",
+        "journal_highlights_for_task",
+    ):
+        getattr(repo, method).return_value = []
+    _ldef = base["journal"].latest_decision_at.return_value
+    if type(_ldef).__name__ in ("MagicMock", "AsyncMock"):
+        base["journal"].latest_decision_at.return_value = datetime.now(UTC)
+    return ChoreographerDeps(**base)
+
+
+@pytest.mark.asyncio
+async def test_create_subtask_propagates_parent_sessions() -> None:
+    """When the choreographer creates a subtask, the parent's session
+    links are auto-attached to it via MessagingService.propagate_sessions_to_subtask.
+    """
+    pm_id = uuid4()
+    parent_id = uuid4()
+    new_task_id = uuid4()
+    parent = MagicMock(
+        id=parent_id,
+        project_id=uuid4(),
+        team="backend",
+        status="in_progress",
+        task_type="planning",
+        sequence=0,
+        assigned_to=pm_id,
+    )
+    new_task = MagicMock(id=new_task_id, status="pending")
+    task_svc = AsyncMock()
+    task_svc.create_subtask.return_value = new_task
+    messaging = AsyncMock()
+    messaging.propagate_sessions_to_subtask.return_value = []
+    deps = _make_deps(task=task_svc, messaging=messaging)
+    c = Choreographer(deps)
+
+    inputs = DelegateInputs(
+        title="Backend slice",
+        description="API + DB",
+        acceptance_criteria=["api works", "schema migrated"],
+        assigned_to="be-dev-1",
+        team="backend",
+        task_type="code",
+        nature="technical",
+        estimated_complexity="medium",
+    )
+    result = await c._create_subtask_from_inputs(pm_id, parent_id, parent, inputs)
+    assert result is new_task
+
+    messaging.propagate_sessions_to_subtask.assert_awaited_once_with(
+        parent_task_id=parent_id,
+        subtask_id=new_task_id,
+        added_by=pm_id,
+    )
+
+
+@pytest.mark.asyncio
+async def test_create_subtask_no_messaging_skips_propagation() -> None:
+    """When messaging dep is None (e.g. lightweight test wiring), the
+    subtask is still created — propagation is a soft enhancement, not a
+    hard requirement."""
+    pm_id = uuid4()
+    parent_id = uuid4()
+    new_task_id = uuid4()
+    parent = MagicMock(
+        id=parent_id,
+        project_id=uuid4(),
+        team="backend",
+        status="in_progress",
+        task_type="planning",
+        sequence=0,
+        assigned_to=pm_id,
+    )
+    new_task = MagicMock(id=new_task_id, status="pending")
+    task_svc = AsyncMock()
+    task_svc.create_subtask.return_value = new_task
+    deps = _make_deps(task=task_svc, messaging=None)
+    c = Choreographer(deps)
+
+    inputs = DelegateInputs(
+        title="Backend slice",
+        description="API + DB",
+        acceptance_criteria=["api works"],
+        assigned_to="be-dev-1",
+        team="backend",
+        task_type="code",
+        nature="technical",
+        estimated_complexity="medium",
+    )
+    # Must not raise even though messaging is None.
+    result = await c._create_subtask_from_inputs(pm_id, parent_id, parent, inputs)
+    assert result is new_task