Extract `is_container()` detection logic from scattered locations
(`config.py`, `voice_mode.py`) into a centralized, cached function in
`hermes_constants.py`. This follows the same pattern as `is_wsl()` and
`is_termux()` — checking `/.dockerenv`, `/run/.containerenv`, and cgroup
markers.
Update gateway status detection (`status.py`, `dump.py`) to use the new
utility and handle Docker/Podman differently from systemd-based systems.
Update setup guidance (`setup.py`) to show Docker restart instructions
when running in a container.
Add Dockerfile.test for CI integration testing and spec.md as a Python
module taste guide for contributors.
