Accept story 36: Enforce Front Matter on All Story Files

Add POST /workflow/stories/create endpoint that auto-assigns story
numbers, generates correct front matter, and writes to upcoming/.
Add slugify_name and next_story_number helpers with full test coverage.
Add frontend createStory API method and types.
Update README to recommend creation API for agents.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/README.md

@@ -45,7 +45,15 @@ When the user asks for a feature, follow this 4-step loop strictly:
 
 ### Step 1: The Story (Ingest)
 *   **User Input:** "I want the robot to dance."
-*   **Action:** Create a file in `stories/upcoming/` (e.g., `stories/upcoming/XX_robot_dance.md`).
+*   **Action:** Create a story via `POST /api/workflow/stories/create` (preferred for agents — guarantees correct front matter and auto-assigns the story number). Alternatively, create a file manually in `stories/upcoming/` (e.g., `stories/upcoming/XX_robot_dance.md`).
+*   **Front Matter (Required):** Every story file MUST begin with YAML front matter containing `name` and `test_plan` fields:
+    ```yaml
+    ---
+    name: Short Human-Readable Story Name
+    test_plan: pending
+    ---
+    ```
+    The `test_plan` field tracks approval status: `pending` → `approved` (after Step 2).
 *   **Move to Current:** Once the story is validated and ready for coding, move it to `stories/current/`.
 *   **Tracking:** Mark Acceptance Criteria as tested directly in the story file as tests are completed.
 *   **Content:**