test: add integration tests for all modules
- Add pytest-asyncio dev dependency and configure asyncio_mode=auto
- Add filterwarnings to suppress third-party PydanticDeprecatedSince20
- Add conftest.py with shared fixtures (project_dir, write_config, etc.)
- Add test_config.py: YAML loading, target type inference, model resolution
- Add test_graph.py: DAG construction, cycle detection, build ordering
- Add test_state.py: hash functions, state persistence, dirty checking
- Add test_builder.py: full build pipeline with FakeProvider, incremental
builds, selective builds, error isolation, dependency cascading
- Add test_providers.py: ImageProvider and TextProvider with mocked clients
- Add test_cli.py: build/clean/graph commands via typer CliRunner
- All 94 tests pass with 0 basedpyright warnings