cleanup: kill dead code, fix signal handler safety

- 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>

src/subconscious/agents/bail-no-competing.sh

@@ -8,15 +8,7 @@
 my_pid_file=$(basename "$1")
 
 for f in pid-*; do
-    [ "$f" = "$my_pid_file" ] && continue
-    [ ! -f "$f" ] && continue
-
-    pid=${f#pid-}
-    if kill -0 "$pid" 2>/dev/null; then
-        exit 1  # another agent is alive, bail
-    else
-        rm -f "$f"  # stale, clean up
-    fi
+    [[ $f != $my_pid_file ]] && exit 1
 done
 
 exit 0