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>

alembic/versions/0066_milestone_body.py

@@ -0,0 +1,32 @@
+"""milestone-as-plan-container: milestones.body holds the plan/design
+
+Revision ID: 0066
+Revises: 0065
+Create Date: 2026-06-14
+
+T3 of plan #819. The milestone becomes the plan container: its `body` holds
+the design/intent/purpose (markdown), `description` stays the one-liner, and
+individual steps live as first-class child tasks (milestone_id) instead of
+checkboxes crammed into a kind=plan task body. start_planning is reworked to
+create a milestone instead of a kind=plan task (hard retirement going forward;
+the 'plan' task_kind enum value stays valid so the historical plan-tasks are
+left readable in place — no body-shredding backfill).
+
+Schema change is just one nullable column; no data migration.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "0066"
+down_revision = "0065"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("milestones", sa.Column("body", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("milestones", "body")