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_mcp_tool_planning.py

@@ -14,13 +14,13 @@ def _bind_user():
 
 @pytest.mark.asyncio
 async def test_start_planning_tool_delegates_to_service():
-    payload = {"task": {"id": 5}, "applicable_rules": [], "subscribed_rulebooks": [],
+    payload = {"milestone": {"id": 5}, "applicable_rules": [], "subscribed_rulebooks": [],
                "applicable_rules_truncated": False, "project_goal": "", "open_task_count": 0}
     with patch("scribe.mcp.tools.tasks.planning_svc.start_planning",
                AsyncMock(return_value=payload)) as mock:
         from scribe.mcp.tools.tasks import start_planning
         out = await start_planning(project_id=3, title="Plan it")
-    assert out["task"]["id"] == 5
+    assert out["milestone"]["id"] == 5
     assert mock.call_args.kwargs == {"user_id": 7, "project_id": 3, "title": "Plan it"}