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>

src/scribe/routes/tasks.py

@@ -99,6 +99,15 @@ async def create_task_route():
     description = data.get("description")
     tags = data.get("tags", [])
 
+    # kind=plan is retired — plans are milestones now (see start_planning).
+    # The 'plan' enum value stays valid for legacy tasks, but new ones can't
+    # be created with it through any path.
+    if data.get("kind") == "plan":
+        return jsonify({
+            "error": "kind=plan is retired — plans are milestones. "
+                     "Use POST /api/tasks/planning to start a plan."
+        }), 400
+
     due_date = parse_iso_date(data.get("due_date"), "due_date")
     if isinstance(due_date, tuple):
         return due_date