feat(05-04): split prod and fullstack compose artifacts

- add bot-only production compose contract
- add health-gated internal fullstack harness

docker-compose.fullstack.yml

@@ -0,0 +1,49 @@
+services:
+  matrix-bot:
+    extends:
+      file: docker-compose.prod.yml
+      service: matrix-bot
+    environment:
+      AGENT_BASE_URL: http://platform-agent:8000
+    depends_on:
+      platform-agent:
+        condition: service_healthy
+
+  platform-agent:
+    build:
+      context: ./external/platform-agent
+      target: development
+      additional_contexts:
+        agent_api: ./external/platform-agent_api
+    environment:
+      PYTHONUNBUFFERED: "1"
+      AGENT_ID: ${AGENT_ID:-matrix-dev}
+      PROVIDER_MODEL: ${PROVIDER_MODEL:-openai/gpt-4o-mini}
+      PROVIDER_URL: ${PROVIDER_URL:-}
+      PROVIDER_API_KEY: ${PROVIDER_API_KEY:-}
+      COMPOSIO_API_KEY: ${COMPOSIO_API_KEY:-}
+    volumes:
+      - ./external/platform-agent/src:/app/src
+      - ./external/platform-agent_api:/agent_api
+      - agents:/workspace
+    command: >
+      sh -lc "
+        mkdir -p /workspace &&
+        chown -R agent:agent /workspace &&
+        exec /app/.venv/bin/uvicorn src.main:app --host 0.0.0.0 --port 8000
+      "
+    ports:
+      - "8000:8000"
+    healthcheck:
+      test:
+        - CMD-SHELL
+        - python -c "import urllib.request; urllib.request.urlopen('http://127.0.0.1:8000/openapi.json', timeout=2).read()"
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 10s
+    restart: unless-stopped
+
+volumes:
+  agents:
+    name: ${SURFACES_SHARED_VOLUME:-surfaces-agents}