fix(api): package api/prompts into the Docker image

The narrator pipeline (M1) reads role prompts from /app/prompts, but the
Dockerfile copied only api/app and docs/schemas — never api/prompts — so
/dm/narrate 500'd with FileNotFoundError on narrator.md in any Docker run.
The M1 live smoke passed only because it ran uvicorn from the local venv,
where ../prompts resolves on the host; the container path was never
exercised until the client HTTP loop drove a real request through compose.

Dockerfile now COPYs api/prompts; compose mounts it :ro so prompt edits
reload live like the app code (prompts are source code, §16). Verified by
building the image and confirming /app/prompts/narrator.md is present.

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

docker-compose.yml

@@ -14,9 +14,11 @@ services:
         required: false
     environment:
       PORT: 8000
-    # Mount source + schemas so edits are live without a rebuild.
+    # Mount source + prompts + schemas so edits are live without a rebuild.
+    # Prompts are source code (§16) — editing a prompt should reload like code.
     volumes:
       - ./api/app:/app/app:ro
+      - ./api/prompts:/app/prompts:ro
       - ./docs/schemas:/app/schemas:ro
     command: >
       uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload