feat(plans): milestone-as-plan-container; retire kind=plan (T3)

The milestone becomes the plan container: a new nullable milestones.body
holds the design/intent (Goal/Approach/Verification) and individual steps
live as first-class child tasks (milestone_id) instead of checkboxes crammed
into one kind=plan task body. start_planning now creates a MILESTONE seeded
with the body template (not a kind=plan task) and returns it with applicable
rules; a new get_milestone MCP tool reads the plan back (body + steps + rules).

kind=plan is hard-retired going forward — start_planning never creates one.
The 'plan' task_kind enum value stays valid so the 11 historical plan-tasks
remain readable in place; no body-shredding backfill (corpus review showed
auto-splitting their checklists into tasks would be lossy: embedded code
blocks, a non-binary [~] state, tables, ID-encoded hierarchy).

- migration 0066: add milestones.body
- model/service/route/MCP: body passthrough on create+update; get_milestone
- server _INSTRUCTIONS: "plan" = milestone w/ body + child step-tasks
- UI: ProjectView shows/edits a milestone's plan body; start_planning expands
  the new milestone and opens its plan editor
- tests updated to the milestone contract + new body/get_milestone coverage

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_services_planning.py

@@ -4,17 +4,19 @@ import pytest
 
 
 @pytest.mark.asyncio
-async def test_start_planning_creates_plan_task_and_returns_rules():
-    fake_note = MagicMock()
-    fake_note.to_dict.return_value = {"id": 5, "title": "Plan it", "task_kind": "plan"}
+async def test_start_planning_creates_milestone_and_returns_rules():
+    # start_planning now creates a MILESTONE (the plan container), not a
+    # kind=plan task; its body holds the seeded design template.
+    fake_milestone = MagicMock()
+    fake_milestone.to_dict.return_value = {"id": 5, "title": "Plan it", "status": "active"}
     applicable = {
         "rules": [{"id": 1, "title": "dev is home", "statement": "...",
                    "topic_title": "git-workflow", "rulebook_title": "FabledSword family"}],
         "truncated": False,
         "subscribed_rulebooks": [{"id": 2, "title": "FabledSword family"}],
     }
-    with patch("scribe.services.planning.notes_svc.create_note",
-               AsyncMock(return_value=fake_note)) as mock_create, \
+    with patch("scribe.services.planning.milestones_svc.create_milestone",
+               AsyncMock(return_value=fake_milestone)) as mock_create, \
          patch("scribe.services.planning.rulebooks_svc.get_applicable_rules",
                AsyncMock(return_value=applicable)), \
          patch("scribe.services.planning.notes_svc.list_notes",
@@ -24,14 +26,13 @@ async def test_start_planning_creates_plan_task_and_returns_rules():
         from scribe.services.planning import start_planning
         out = await start_planning(user_id=7, project_id=3, title="Plan it")
 
-    # Created a plan-task (status set => task, kind=plan)
+    # Created a milestone with the seeded plan-body template.
     kwargs = mock_create.call_args.kwargs
-    assert kwargs["task_kind"] == "plan"
-    assert kwargs["status"] == "todo"
     assert kwargs["project_id"] == 3
+    assert kwargs["status"] == "active"
     assert "## Goal" in kwargs["body"]  # seeded template
     # Returned shape
-    assert out["task"]["id"] == 5
+    assert out["milestone"]["id"] == 5
     assert out["applicable_rules"][0]["title"] == "dev is home"
     assert out["subscribed_rulebooks"] == [{"id": 2, "title": "FabledSword family"}]
     assert out["open_task_count"] == 3