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/models/milestone.py

@@ -13,6 +13,10 @@ class Milestone(Base, TimestampMixin, SoftDeleteMixin):
     project_id: Mapped[int] = mapped_column(Integer, ForeignKey("projects.id", ondelete="CASCADE"))
     title: Mapped[str] = mapped_column(Text, default="")
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # The plan: design/intent/purpose (markdown). The milestone is the plan
+    # container; its steps live as first-class child tasks (milestone_id), not
+    # as checkboxes in this body. `description` stays the one-line summary.
+    body: Mapped[str | None] = mapped_column(Text, nullable=True)
     status: Mapped[str] = mapped_column(Text, default="active")
     order_index: Mapped[int] = mapped_column(Integer, default=0)
 
@@ -23,6 +27,7 @@ class Milestone(Base, TimestampMixin, SoftDeleteMixin):
             "project_id": self.project_id,
             "title": self.title,
             "description": self.description,
+            "body": self.body,
             "status": self.status,
             "order_index": self.order_index,
             "created_at": self.created_at.isoformat(),