fix: prevent systemd restart loop from stale/colliding PID files

[Bartok9]

Fixes #576

Problem

When Hermes is run under systemd, the service can enter restart loops due to:

1. Stale PID files: If the gateway crashes without cleanup (e.g., SIGKILL), the PID file remains
2. PID collision: A different process may reuse the same PID number, causing false "Gateway already running" errors

Solution

1. Robust PID verification

Enhanced get_running_pid() to verify the process is actually a hermes gateway, not just any process with that PID. The new _is_hermes_gateway_process() helper:

• On Linux: checks /proc/{pid}/cmdline directly (fast, no subprocess)
• On macOS: uses ps command

2. Added --replace flag

New hermes gateway run --replace option that:

• Terminates any existing gateway gracefully
• Cleans up stale PID files
• Enables idempotent startup for service managers

3. Improved service templates

systemd unit:

• Uses --replace by default
• Added ExecStop for clean shutdown
• Added StartLimitIntervalSec/StartLimitBurst to prevent aggressive restart loops
• Changed After=network.target to After=network-online.target
• Set RestartSec=15 (was 10)
• Explicitly sets HERMES_HOME environment variable

launchd plist:

• Uses --replace by default
• Added ThrottleInterval=15
• Explicitly sets HERMES_HOME environment variable

4. Better error messages

When duplicate instance is detected, the error message now suggests:

• hermes gateway run --replace for force takeover
• hermes gateway install --force for service reinstallation

Testing

Added tests/gateway/test_status.py with tests for:

• Nonexistent PID handling
• Stale PID file cleanup
• PID collision detection (core fix)
• Basic write/remove operations

All tests pass.

For users experiencing this issue

After updating, reinstall the service to get the new template:

hermes gateway install --force
hermes gateway start

Or manually start with:

hermes gateway run --replace

[teknium1]

Fixed via PR #614 (39ee351) which adds a --replace flag with proper SIGTERM→SIGKILL escalation. Your PID collision detection idea is solid defense-in-depth — we may revisit that as a follow-up. Thanks @Bartok9!