Bug Description

When the gateway is managed by a macOS LaunchAgent with KeepAlive: true, a SIGUSR1 in-process restart causes an infinite respawn loop.

Steps to Reproduce

1. Install OpenClaw as a LaunchAgent service (openclaw gateway start creates plist with KeepAlive: true)
2. Trigger any restart via SIGUSR1 (e.g. /restart, config change, gateway restart tool)
3. Observe ~10-second loop of "Gateway failed to start: gateway already running" errors

What Happens

1. SIGUSR1 handler does restart mode: full process restart (spawned pid XXXX) — spawns a new child process and the original PID exits
2. LaunchAgent sees the original PID exit → thinks the service crashed → spawns another gateway process
3. New spawn finds port already in use (child from step 1 owns it) → fails → exits
4. LaunchAgent respawns again → repeat every ~10 seconds indefinitely

Impact

• Webchat clients experience repeated disconnect/reconnect cycles
• Log fills with "Port 18789 is already in use" / "gateway already running" errors at ~10s intervals
• The loop persists until manually stopped or the webchat client re-establishes and stabilizes

Environment

• OpenClaw 2026.2.15
• macOS 26.2 (arm64), M4 Pro Mac mini
• LaunchAgent plist at ~/Library/LaunchAgents/ai.openclaw.gateway.plist

Log Evidence

07:19:57.255Z gateway received SIGUSR1; restarting
07:19:57.405Z gateway restart mode: full process restart (spawned pid 57144)
07:20:07.309Z Gateway failed to start: gateway already running (pid 57144); lock timeout after 5000ms
07:20:18.352Z Gateway failed to start: gateway already running (pid 57144); lock timeout after 5000ms
... (repeats every ~10s)

Suggested Fix

When running under a LaunchAgent (or any process supervisor), SIGUSR1 should either:

1. Restart in-place (exec into new process, preserving PID) instead of fork+exit
2. Detect the supervisor and use launchctl kickstart to do a clean service restart
3. Not exit the parent process — keep the original PID alive as a thin wrapper

Option 1 (exec-style restart) would be the most compatible with any process supervisor (systemd, launchd, etc.).