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>

frontend/src/types/task.ts

@@ -5,8 +5,18 @@ export interface TaskListResponse {
   total: number;
 }
 
+// start_planning now creates a MILESTONE (the plan container), not a kind=plan
+// task. The milestone's `body` holds the design; steps live as child tasks.
 export interface StartPlanningResult {
-  task: import("./note").Note;
+  milestone: {
+    id: number;
+    project_id: number;
+    title: string;
+    description: string | null;
+    body: string | null;
+    status: string;
+    order_index: number;
+  };
   applicable_rules: {
     id: number;
     title: string;