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>

src/scribe/routes/tasks.py

@@ -6,6 +6,7 @@ from scribe.auth import login_required, get_current_user_id
 from scribe.models.note import TaskPriority, TaskStatus
 from scribe.routes.utils import not_found, parse_iso_date, parse_pagination
 from scribe.services.access import can_write_note
+from scribe.services import placement as placement_svc
 from scribe.services import systems as systems_svc
 from scribe.services.notes import (
     create_note,
@@ -151,6 +152,7 @@ async def create_task_route():
         await systems_svc.set_record_systems(uid, task.id, data["system_ids"])
     out = task.to_dict()
     out["systems"] = [s.to_dict() for s in await systems_svc.list_record_systems(uid, task.id)]
+    await placement_svc.attach_placement(uid, out, task)
     return jsonify(out), 201
 
 
@@ -267,6 +269,7 @@ async def update_task_route(task_id: int):
         await systems_svc.set_record_systems(uid, task_id, data["system_ids"])
     out = task.to_dict()
     out["systems"] = [s.to_dict() for s in await systems_svc.list_record_systems(uid, task_id)]
+    await placement_svc.attach_placement(uid, out, task)
     return jsonify(out)