refactor: migrate extension system to use deep directory paths based on module and qualname

- Change @extensible decorator to generate extension paths from full module and qualname hierarchies instead of flattened names
- Update extension path format from `{module}_{qualname}_{start|end}` to `_functions/<module>/<qualname>/{start|end}`
- Move all extension files to new deep directory structure under `_functions/` to match new path format
- Replace PathSpec.from_lines(GitWildMatchPattern, ...) with PathSpec.from_lines

plugins/_model_config/extensions/python/startup_migration/_10_migrate_model_config.py

@@ -28,7 +28,7 @@ class MigrateModelConfig(Extension):
         "browser_model_rl_output", "browser_model_kwargs", "browser_http_headers",
     ]
 
-    async def execute(self, **kwargs):
+    def execute(self, **kwargs):
         # Check if global plugin config already exists
         global_config_path = files.get_abs_path("plugins/_model_config/config.json")
         if os.path.exists(global_config_path):