Add probeExternalOpenCode() to detect a running OpenCode instance via
GET /global/health before attempting to spawn a new one. When an external
server is detected (or OPENCODE_SKIP_START is set), the isExternalOpenCode
flag prevents gracefulShutdown() and restartOpenCode() from killing the
user's independent OpenCode process.
- Probe configured port, then default 4096 on startup
- Use /global/health + body.healthy === true for reliable detection
- Skip kill -9 on shutdown/restart when server is externally owned
- Re-probe (instead of kill + respawn) on restart for external servers