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/services/milestones.py

@@ -16,6 +16,7 @@ async def create_milestone(
     project_id: int,
     title: str,
     description: str | None = None,
+    body: str | None = None,
     order_index: int = 0,
     status: str = "active",
 ) -> Milestone:
@@ -25,6 +26,7 @@ async def create_milestone(
             project_id=project_id,
             title=title,
             description=description,
+            body=body,
             status=status,
             order_index=order_index,
         )

src/scribe/services/planning.py

@@ -1,31 +1,37 @@
-"""Planning service — start_planning aggregates plan-task creation with the
-project's applicable Rulebook rules and a little context, so planning happens
-in Scribe and rules surface at the planning moment.
+"""Planning service — start_planning creates a MILESTONE seeded as the plan
+container, surfacing the project's applicable Rulebook rules at the planning
+moment so rules land before any work.
+
+The milestone IS the plan: its `body` holds the design/intent (Goal/Approach/
+Verification), and the individual steps live as first-class child tasks
+(milestone_id) rather than checkboxes crammed into one body. The legacy
+kind=plan task is retired going forward — start_planning never creates one.
 """
 from __future__ import annotations
 
+from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
 from scribe.services import projects as projects_svc
 from scribe.services import rulebooks as rulebooks_svc
 
+# The plan body template — design only. Steps are NOT checkboxes here; each
+# step becomes its own child task under this milestone (status, work-logs,
+# priority of its own).
 PLAN_TEMPLATE = """## Goal
 
 ## Approach
 
-## Steps
-- [ ]
-
 ## Verification
 """
 
 
 async def start_planning(user_id: int, project_id: int, title: str) -> dict:
-    """Create a plan-task seeded with the plan template and return it with the
+    """Create a milestone seeded as a plan container and return it with the
     project's applicable rules + brief context.
 
     Returns:
         {
-          "task": <task dict>,
+          "milestone": <milestone dict>,
           "applicable_rules": [...],
           "subscribed_rulebooks": [...],
           "applicable_rules_truncated": bool,
@@ -37,13 +43,12 @@ async def start_planning(user_id: int, project_id: int, title: str) -> dict:
     if project is None:
         raise ValueError(f"project {project_id} not found")
 
-    note = await notes_svc.create_note(
+    milestone = await milestones_svc.create_milestone(
         user_id,
+        project_id=project_id,
         title=title,
         body=PLAN_TEMPLATE,
-        status="todo",
-        task_kind="plan",
-        project_id=project_id,
+        status="active",
     )
 
     applicable = await rulebooks_svc.get_applicable_rules(
@@ -54,7 +59,7 @@ async def start_planning(user_id: int, project_id: int, title: str) -> dict:
     )
 
     return {
-        "task": note.to_dict(),
+        "milestone": milestone.to_dict(),
         "applicable_rules": applicable["rules"],
         "subscribed_rulebooks": applicable["subscribed_rulebooks"],
         "applicable_rules_truncated": applicable["truncated"],