Support base digest pinning and OCI subject linkage for adapters

[gorkem]

Describe the problem you're trying to solve
KitOps currently links adapters to base models via Kitfile metadata but does not record the exact base digest nor expose the relationship at the OCI level. This issue adds two enhancements to improve reliability and interoperability: capture and record the base’s immutable digest when packing adapters, and optionally set the OCI subject field in the adapter manifest to point at the base manifest digest.

Describe the solution you'd like

1. Digest pinning - On kit pack for a Lora adapter, resolve the base reference (model.path) to its immutable OCI manifest digest
   • Record that digest in Config/Kitfile
2. OCI subject support
   • When generating the adapter’s OCI manifest, optionally set the subject field to reference the base manifest digest, for example:

     "subject": {
       "mediaType": "application/vnd.oci.image.manifest.v1+json",
       "digest": "sha256:<base-manifest-digest>"
     }

[arnab2001]

Willing to pick it up if its still needed