fix(agent): resolve ModuleNotFoundError for backend, add E2E test infra

- Dockerfile.agent: fix CMD (python -m src.agent.run), use backend/requirements.txt,
  minimal COPY (only src.agent + src.core.cot_logger), add GRACE contract
- docker-compose.yml: SERVICE_TOKEN_SECRET -> SERVICE_JWT (match code)
- docker-compose.enterprise-clean.yml: same env var fix
- docker/.env.agent.example: same env var fix
- build.sh: same env var fix
- chore: semantics-testing SKILL.md, backend tests, pyproject.toml

build.sh

@@ -304,7 +304,7 @@ services:
       LLM_MODEL: \${LLM_MODEL:-gpt-4o}
       FASTAPI_URL: http://backend:8000
       JWT_SECRET: \${JWT_SECRET:-super-secret-key}
-      SERVICE_TOKEN_SECRET: \${SERVICE_TOKEN_SECRET:-agent-service-secret}
+      SERVICE_JWT: \${SERVICE_JWT:-agent-service-secret}
       DATABASE_URL: postgresql+psycopg2://\${POSTGRES_USER:-postgres}:\${POSTGRES_PASSWORD:?Set POSTGRES_PASSWORD}@\${POSTGRES_HOST:?Set POSTGRES_HOST}:\${POSTGRES_PORT:-5432}/\${POSTGRES_DB:-ss_tools}
       GRADIO_SERVER_PORT: 7860
     ports: