chore(plans): make kind=plan retirement consistent across MCP, REST, UI, skills

Audit of the plugin + MCP surface after milestone-as-plan (T3): every path
that could still create a kind=plan task or describe the old plan-task model
is now aligned with the hard-retire decision.

- create_task (MCP + REST POST /api/tasks): reject kind=plan with a message
  pointing to start_planning. The 'plan' enum value stays valid so legacy
  plan-tasks remain readable; update paths never touch kind, so they round-trip.
- create_task / get_task docstrings: 'plan' dropped from creatable kinds;
  get_task's rules-augmentation noted as legacy-only (get_milestone for new plans).
- skills/writing-plans: rewritten for milestone-as-plan (body = design, steps =
  child tasks, get_milestone to read back).
- skills/using-scribe: "plans live in milestones via start_planning", not kind=plan.
- TaskEditorView Kind selector: offers Work/Issue; "Plan (legacy)" shown only
  when the loaded task is already kind=plan (display round-trip).
- test: create_task rejects kind=plan.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_mcp_tool_tasks.py

@@ -113,6 +113,16 @@ async def test_create_task_passes_status():
     assert mock.call_args.kwargs["status"] == "todo"
 
 
+@pytest.mark.asyncio
+async def test_create_task_rejects_retired_plan_kind():
+    """kind=plan is hard-retired — plans are milestones (start_planning)."""
+    mock = AsyncMock()
+    with patch("scribe.mcp.tools.tasks.notes_svc.create_note", mock):
+        with pytest.raises(ValueError, match="kind=plan is retired"):
+            await create_task(title="x", kind="plan")
+    assert not mock.called  # never reached the create
+
+
 @pytest.mark.asyncio
 async def test_create_task_priority_empty_becomes_none():
     fake = _fake_task()