- Remove unused now_secs(), parse_json_response, any_alive, Regex import
- Signal handler: replace Mutex with AtomicPtr<c_char> for signal safety
(Mutex::lock in a signal handler can deadlock if main thread holds it)
- PidGuard Drop reclaims the leaked CString; signal handler just unlinks
- scan_pid_files moved to knowledge.rs as pub helper
- setup_agent_state calls scan_pid_files to clean stale pids on startup
Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>
