feat(409): a task write returns where the task sits (#4010)

Step 1 of milestone 409 "Response shapes". An agent reporting finished work
is asked to say which milestone it belongs to, which step of how many, and
what is next. Without those facts to hand it reconstructs them, and a
reconstruction reads exactly like the truth when it is wrong.

create_task and update_task (MCP) and the REST create/update task routes now
return a placement block: project; and for a task in a milestone, the
milestone, position {step, of}, progress {completed, total, pct} and next
(the next open step, falling back to the earliest open one before it).

- Step order is creation order, not get_milestone listing order, which
  reshuffles on every update.
- Siblings are read through readable_notes_clause; position and progress
  are computed over that same readable set, so a collaborator is never shown
  a step title they cannot open, and a note share alone reveals no plan.
- Fail-open and omitted when empty, like every in-band decoration.
- _no_embedding moves into conftest as one opt-in fixture for both
  integration modules that need it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/test_integration_record_batch.py

@@ -7,7 +7,6 @@ properties of the sequence and the transaction, so they are measured against
 the real ones.
 """
 import asyncio
-from unittest.mock import MagicMock, patch
 
 import pytest
 import pytest_asyncio
@@ -22,16 +21,7 @@ from scribe.services.planning import start_planning
 from scribe.services.record_batch import BatchItem, create_batch
 from tests.helpers import ensure_user
 
-pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
-
-
-@pytest.fixture(autouse=True)
-def _no_embedding():
-    # embed_note detaches a task that loads the embedding model; this lane is
-    # about ids and transactions, and a background model load would outlive
-    # the test's event loop.
-    with patch("scribe.services.notes.embed_note", MagicMock()):
-        yield
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine", "_no_embedding")]
 
 
 @pytest_asyncio.fixture