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

.opencode/skills/semantics-testing/SKILL.md

@@ -104,16 +104,28 @@ class TestDashboardMigration:
 
 ### Running tests
 ```bash
-# All backend tests
+# All backend tests (integration tests skipped by default)
 cd backend && source .venv/bin/activate && python -m pytest -v
 
 # Specific test file
 python -m pytest tests/test_migration.py -v
 
+# Include integration tests (PostgreSQL/Superset Testcontainers)
+python -m pytest --run-integration
+
+# Run only integration tests
+python -m pytest tests/integration/ --run-integration
+
 # With coverage
 python -m pytest --cov=src --cov-report=term-missing
 ```
 
+**Integration tests** (`tests/integration/`) use Testcontainers (PostgreSQL 16, Superset 4.1.2)
+and require Docker. They are **skipped by default** — pass `--run-integration` to enable.
+The `--run-integration` flag is registered in `backend/tests/conftest.py` via `pytest_addoption`;
+skip logic lives in `backend/tests/integration/conftest.py` via `pytest_collection_modifyitems`.
+See also: `backend/pyproject.toml` `[tool.pytest.ini_options] markers` for the registered marker.
+
 ## VII. SVELTE / VITEST CONVENTIONS
 
 ### Test file structure