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/mcp/tools/tasks.py

@@ -233,14 +233,24 @@ async def add_task_log(task_id: int, content: str) -> dict:
 async def start_planning(project_id: int, title: str) -> dict:
     """Begin a plan in Scribe (the preferred home for plans — not a local .md file).
 
-    Creates a plan-task (a task with kind=plan) seeded with a plan template under
-    the given project, and returns it together with the project's applicable
-    Rulebook rules and brief context. Maintain the plan afterwards with the normal
-    task tools (update_task to edit the body, add_task_log to record progress).
+    Creates a MILESTONE that IS the plan: its `body` is seeded with a design
+    template (Goal/Approach/Verification) under the given project, and the call
+    returns it together with the project's applicable Rulebook rules and brief
+    context. The milestone is the plan container — the individual steps live as
+    first-class child tasks under it, not as checkboxes in the body.
+
+    Afterwards:
+    - Edit the plan/design with update_milestone(milestone_id, body=...).
+    - Create each step as its own task with create_task(milestone_id=<this id>);
+      track it with status + add_task_log. Do NOT put steps as checkboxes in the
+      milestone body.
+
+    (kind=plan tasks are retired — use this instead. Existing historical
+    plan-tasks remain readable but new planning goes through milestones.)
 
     Args:
         project_id: The project this plan is for.
-        title: A short title for the plan.
+        title: A short title for the plan/milestone.
     """
     uid = current_user_id()
     return await planning_svc.start_planning(