Function default_plugin_dir 

pub fn default_plugin_dir() -> PathBuf

The directory the registry scans for plugin sub-directories.

LEAN_CTX_PLUGINS_DIR (the root containing plugin folders) overrides the default so containers, CI, and tests can point at an isolated location. Note this is distinct from the per-hook LEAN_CTX_PLUGIN_DIR the executor sets for a single plugin’s child process.