Dev AI config: env var overrides in config_reader, LM Studio via .env

config_reader.py now merges environment variables (AI_PROVIDER,
LMSTUDIO_BASE_URL, LMSTUDIO_API_KEY, LMSTUDIO_MODEL, OLLAMA_*,
ANTHROPIC_*) on top of the JSON config file, so the dev .env file
can pin the AI connection without writing to the shared config volume.

docker-compose.dev.yml loads features/doc-service/.env (gitignored)
into the doc-service container so the token is never committed.

.env.example updated with all supported override variables and comments.

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

docker-compose.dev.yml

@@ -25,5 +25,6 @@ services:
 
   doc-service:
     command: sh scripts/start_dev.sh
+    env_file: ./features/doc-service/.env   # gitignored — holds local AI credentials
     volumes:
       - ./features/doc-service:/app