Delete pid_utils.py and migrate to Process model

DELETED:
- workers/pid_utils.py (-192 lines) - replaced by Process model methods

SIMPLIFIED:
- crawls/models.py Crawl.cleanup() (80 lines -> 10 lines)
- hooks.py: deleted process_is_alive() and kill_process() (-45 lines)

UPDATED to use Process model:
- core/models.py: Snapshot.cleanup() and has_running_background_hooks()
- machine/models.py: Binary.cleanup()
- workers/worker.py: Worker.on_startup/shutdown, get_running_workers, start
- workers/orchestrator.py: Orchestrator.on_startup/shutdown, is_running

All subprocess management now uses:
- Process.current() for registering current process
- Process.get_running() / get_running_count() for querying
- Process.cleanup_stale_running() for cleanup
- safe_kill_process() for validated PID killing

Total line reduction: ~250 lines

archivebox/machine/models.py

@@ -449,7 +449,7 @@ class Binary(ModelWithHealthStats):
         since installations are foreground, but included for consistency).
         """
         from pathlib import Path
-        from archivebox.hooks import kill_process
+        from archivebox.misc.process_utils import safe_kill_process
 
         output_dir = self.OUTPUT_DIR
         if not output_dir.exists():
@@ -460,8 +460,8 @@ class Binary(ModelWithHealthStats):
             if not plugin_dir.is_dir():
                 continue
             pid_file = plugin_dir / 'hook.pid'
-            if pid_file.exists():
-                kill_process(pid_file)
+            cmd_file = plugin_dir / 'cmd.sh'
+            safe_kill_process(pid_file, cmd_file)
 
 
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