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>

src/scribe/routes/milestones.py

@@ -62,6 +62,7 @@ async def create_milestone_route(project_id: int):
         project_id,
         title=data["title"],
         description=data.get("description"),
+        body=data.get("body"),
         order_index=data.get("order_index", 0),
         status=status,
     )
@@ -92,7 +93,7 @@ async def update_milestone_route(project_id: int, milestone_id: int):
     if milestone is None:
         return not_found("Milestone")
     data = await request.get_json()
-    allowed = {"title", "description", "status", "order_index"}
+    allowed = {"title", "description", "body", "status", "order_index"}
     fields = {k: v for k, v in data.items() if k in allowed}
     if "status" in fields and fields["status"] not in ("active", "done"):
         return jsonify({"error": "status must be 'active' or 'done'"}), 400