fix: shut down gracefully on SIGTERM

[nanomad]

In a container the gateway runs as PID 1. Python installs no SIGTERM handler, and the kernel ignores a default-action SIGTERM for PID 1, so docker stop waited 10 s and then sent SIGKILL. The finally that closes the SAIC API client (#479) and other cleanup never ran.

• Add run_until_terminated() in src/shutdown.py. On SIGTERM it cancels the main task, so finally blocks run, and the process exits with code 0. Exceptions and other cancellations (such as Ctrl-C) are not swallowed. If a handler can't be installed on the platform, it is skipped.
• Use it in src/main.py.
• Add .git/, .claude/ and tests/ to .dockerignore to keep them out of the build context. README.md stays, because pyproject.toml declares it as the readme.

Tested: five unit tests (including a real SIGTERM), and a subprocess run where SIGTERM runs the cleanup and exits with code 0. Ruff, mypy and the full suite pass (323 tests). Not tested: a real docker stop against the built image.