dsh-bundle-manager: DSH Third-Party Plugin Runtime Mount Manager

# Introduction The philosophy of DeepSeek Harness (DSH) is "everything is a plugin." However, following the official design, bundles written to `dsh.profile.bundles` are loaded upon process startup and remain resident. To temporarily disable a third-party plugin, one must modify the profile manifest and restart the process. Simultaneously, DSH has a key architectural requirement: optional third-party plugins can only be placed in `dependencies` and must not be written into `dsh.profile.bundles`. In other words, the official static layer does not provide a location for enabling or disabling these plugins at runtime. `dsh-bundle-manager` fills this gap: it mounts within the process via the Loader API.