Add container detection utility to hermes_constants

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.

hermes_cli/dump.py

@@ -44,6 +44,16 @@ def _redact(value: str) -> str:
 def _gateway_status() -> str:
     """Return a short gateway status string."""
     if sys.platform.startswith("linux"):
+        from hermes_constants import is_container
+        if is_container():
+            try:
+                from hermes_cli.gateway import find_gateway_pids
+                pids = find_gateway_pids()
+                if pids:
+                    return f"running (docker, pid {pids[0]})"
+                return "stopped (docker)"
+            except Exception:
+                return "stopped (docker)"
         try:
             from hermes_cli.gateway import get_service_name
             svc = get_service_name()