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/mcp/tools/tasks.py

@@ -63,9 +63,10 @@ async def get_task(task_id: int) -> dict:
     """Fetch a single Scribe task by ID.
 
     Returns id, title, body, status, priority, tags, project_id, milestone_id,
-    parent_id, parent_title, due_date, created_at, updated_at. For kind=plan
-    tasks, the response also includes applicable_rules + subscribed_rulebooks
-    from the task's project's rulebook subscriptions.
+    parent_id, parent_title, due_date, created_at, updated_at. For legacy
+    kind=plan tasks, the response also includes applicable_rules +
+    subscribed_rulebooks from the task's project's rulebook subscriptions (new
+    plans are milestones — use get_milestone for those).
     """
     uid = current_user_id()
     note = await notes_svc.get_note(uid, task_id)
@@ -79,6 +80,8 @@ async def get_task(task_id: int) -> dict:
             parent_title = parent.title
     data["parent_title"] = parent_title
 
+    # Legacy kind=plan tasks predate milestone-as-plan; still surface their
+    # project's rules on read so the historical plans stay useful.
     if data.get("task_kind") == "plan" and note.project_id:
         applicable = await rulebooks_svc.get_applicable_rules(
             project_id=note.project_id, user_id=uid,
@@ -116,15 +119,23 @@ async def create_task(
         milestone_id: Place within a project milestone (0 = no milestone).
         parent_id: Make this a sub-task of another task (0 = top-level).
         tags: List of plain-string tags without # prefix.
-        kind: 'work' (default), 'plan', or 'issue'. An issue is corrective work —
-            a problem you fixed or are fixing; record symptom → root cause → fix
-            in the body. Prefer start_planning to create plans.
+        kind: 'work' (default) or 'issue'. An issue is corrective work — a
+            problem you fixed or are fixing; record symptom → root cause → fix
+            in the body. (Plans are milestones now — call start_planning to begin
+            a plan; 'plan' is not a valid kind here.)
         system_ids: Ids of the project's Systems (reusable subsystem/area
             objects; see list_systems / create_system) to associate this task with.
         arose_from_id: For an issue, the id of the task/feature it arose from
             (provenance). 0 = none.
     """
     uid = current_user_id()
+    if kind == "plan":
+        raise ValueError(
+            "kind=plan is retired — a plan is now a milestone. Call "
+            "start_planning(project_id, title) to begin a plan (it creates the "
+            "milestone + seeds the design), then create each step as its own "
+            "task with create_task(milestone_id=<that milestone>)."
+        )
     note = await notes_svc.create_note(
         uid,
         title=title,

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