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/milestones.py

@@ -13,32 +13,75 @@ from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
 from scribe.services import milestones as milestones_svc
+from scribe.services import notes as notes_svc
+from scribe.services import rulebooks as rulebooks_svc
 from scribe.services import trash as trash_svc
 
 
 async def list_milestones(project_id: int) -> dict:
     """List milestones for a Scribe project, ordered by order_index.
 
-    Returns id, title, description, status (active/done), order_index,
-    and task counts.
+    Returns id, title, description, body (the plan/design), status
+    (active/done), order_index, and task counts.
     """
     uid = current_user_id()
     rows = await milestones_svc.get_project_milestone_summary(uid, project_id)
     return {"milestones": rows}
 
 
+async def get_milestone(milestone_id: int) -> dict:
+    """Fetch a milestone (the plan container) with its step-tasks and rules.
+
+    A milestone IS a plan: its `body` holds the design/intent, and its steps
+    are the child tasks listed here. Use this to read a plan top-to-bottom —
+    the body for the design, `steps` for the trackable units of work. Mirrors
+    the planning context that start_planning returns (applicable rules), so the
+    rules surface again on recall.
+
+    Returns: milestone (incl. body), progress, steps (its tasks ordered by
+    status then update), and applicable_rules / subscribed_rulebooks.
+    """
+    uid = current_user_id()
+    milestone = await milestones_svc.get_milestone(uid, milestone_id)
+    if milestone is None:
+        raise ValueError(f"milestone {milestone_id} not found")
+    progress = await milestones_svc.get_milestone_progress(milestone_id)
+    steps, _ = await notes_svc.list_notes(
+        uid, is_task=True, milestone_id=milestone_id, sort="status", limit=200,
+    )
+    applicable = await rulebooks_svc.get_applicable_rules(
+        project_id=milestone.project_id, user_id=uid,
+    )
+    out = milestone.to_dict()
+    out.update(progress)
+    return {
+        "milestone": out,
+        "steps": [t.to_dict() for t in steps],
+        "applicable_rules": applicable["rules"],
+        "subscribed_rulebooks": applicable["subscribed_rulebooks"],
+        "applicable_rules_truncated": applicable["truncated"],
+    }
+
+
 async def create_milestone(
     project_id: int,
     title: str,
     description: str = "",
+    body: str = "",
     status: str = "active",
 ) -> dict:
     """Create a milestone within a Scribe project.
 
+    A milestone can serve as a plan container — put the design/intent in `body`
+    and track each step as a child task (create_task(milestone_id=...)). For a
+    fresh plan, prefer start_planning, which seeds the body template + surfaces
+    the project's rules.
+
     Args:
         project_id: The project this milestone belongs to (required).
         title: Milestone name (required).
-        description: Optional description of what this milestone covers.
+        description: Optional one-line summary of what this milestone covers.
+        body: Optional plan/design (markdown) — the milestone's full plan text.
         status: active (default) or done.
     """
     uid = current_user_id()
@@ -47,6 +90,7 @@ async def create_milestone(
         project_id=project_id,
         title=title,
         description=description or None,
+        body=body or None,
         status=status,
     )
     return milestone.to_dict()
@@ -57,6 +101,7 @@ async def update_milestone(
     milestone_id: int,
     title: str = "",
     description: str = "",
+    body: str = "",
     status: str = "",
     order_index: int = -1,
 ) -> dict:
@@ -67,7 +112,8 @@ async def update_milestone(
             ownership scoping is enforced by user_id at the service layer).
         milestone_id: ID of the milestone to update.
         title: New title, or omit to leave unchanged.
-        description: New description, or omit to leave unchanged.
+        description: New one-line summary, or omit to leave unchanged.
+        body: New plan/design (markdown), or omit to leave unchanged.
         status: New status — active or done.
         order_index: New display position (0-based). Use -1 to leave unchanged.
     """
@@ -77,6 +123,8 @@ async def update_milestone(
         fields["title"] = title
     if description:
         fields["description"] = description
+    if body:
+        fields["body"] = body
     if status:
         fields["status"] = status
     if order_index >= 0:
@@ -101,6 +149,7 @@ async def delete_milestone(milestone_id: int) -> dict:
 def register(mcp) -> None:
     for fn in (
         list_milestones,
+        get_milestone,
         create_milestone,
         update_milestone,
         delete_milestone,