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>

tests/test_mcp_tool_milestones.py

@@ -5,7 +5,7 @@ import pytest
 
 from scribe.mcp._context import _user_id_ctx
 from scribe.mcp.tools.milestones import (
-    list_milestones, create_milestone, update_milestone,
+    list_milestones, get_milestone, create_milestone, update_milestone,
 )
 
 
@@ -57,6 +57,64 @@ async def test_create_milestone_empty_description_becomes_none():
     assert mock.call_args.kwargs["description"] is None
 
 
+@pytest.mark.asyncio
+async def test_create_milestone_passes_body_through():
+    """The milestone-as-plan body is forwarded to the service."""
+    m = _fake_ms(id=5)
+    mock = AsyncMock(return_value=m)
+    with patch("scribe.mcp.tools.milestones.milestones_svc.create_milestone", mock):
+        await create_milestone(project_id=1, title="t", body="## Goal\n\nship")
+    assert mock.call_args.kwargs["body"] == "## Goal\n\nship"
+
+
+@pytest.mark.asyncio
+async def test_create_milestone_empty_body_becomes_none():
+    m = _fake_ms()
+    mock = AsyncMock(return_value=m)
+    with patch("scribe.mcp.tools.milestones.milestones_svc.create_milestone", mock):
+        await create_milestone(project_id=1, title="t", body="")
+    assert mock.call_args.kwargs["body"] is None
+
+
+@pytest.mark.asyncio
+async def test_update_milestone_sends_body():
+    m = _fake_ms()
+    mock = AsyncMock(return_value=m)
+    with patch("scribe.mcp.tools.milestones.milestones_svc.update_milestone", mock):
+        await update_milestone(project_id=1, milestone_id=5, body="new plan")
+    assert mock.call_args.kwargs == {"body": "new plan"}
+
+
+@pytest.mark.asyncio
+async def test_get_milestone_returns_body_steps_and_rules():
+    m = _fake_ms(id=5, project_id=3, body="## Goal")
+    step = MagicMock()
+    step.to_dict.return_value = {"id": 9, "title": "step 1", "status": "todo"}
+    applicable = {"rules": [{"id": 1, "title": "r"}], "truncated": False,
+                  "subscribed_rulebooks": [{"id": 2, "title": "rb"}]}
+    with patch("scribe.mcp.tools.milestones.milestones_svc.get_milestone",
+               AsyncMock(return_value=m)), \
+         patch("scribe.mcp.tools.milestones.milestones_svc.get_milestone_progress",
+               AsyncMock(return_value={"total": 1, "completed": 0, "pct": 0.0})), \
+         patch("scribe.mcp.tools.milestones.notes_svc.list_notes",
+               AsyncMock(return_value=([step], 1))), \
+         patch("scribe.mcp.tools.milestones.rulebooks_svc.get_applicable_rules",
+               AsyncMock(return_value=applicable)):
+        out = await get_milestone(milestone_id=5)
+    assert out["milestone"]["body"] == "## Goal"
+    assert out["milestone"]["total"] == 1
+    assert out["steps"] == [{"id": 9, "title": "step 1", "status": "todo"}]
+    assert out["applicable_rules"] == [{"id": 1, "title": "r"}]
+
+
+@pytest.mark.asyncio
+async def test_get_milestone_raises_when_not_found():
+    with patch("scribe.mcp.tools.milestones.milestones_svc.get_milestone",
+               AsyncMock(return_value=None)):
+        with pytest.raises(ValueError, match="milestone 999 not found"):
+            await get_milestone(milestone_id=999)
+
+
 @pytest.mark.asyncio
 async def test_update_milestone_only_sends_non_default_fields():
     m = _fake_ms()