Refactor: use oras.CopyGraph/Copy, add storeAdapter, fix review findings

Replace ~350 lines of manual staging/storing code with ORAS's built-in
oras.CopyGraph (push) and oras.Copy (pull). Introduce storeAdapter to
bridge the local Store with ORAS content interfaces.

Key changes:
- storeAdapter wraps Store, routing by media type to PutManifest/PutBlob
- validatingTarget enforces size/count limits on the pull path
- oras.CopyGraph handles recursive graph traversal for push
- oras.Copy handles recursive pull with automatic tagging
- Removed SetPlainHTTP mutable setter (WithPlainHTTP option suffices)
- Removed all manual staging/storing helpers

Review findings addressed:
- Compile-time interface assertions for storeAdapter and validatingTarget
- Exists distinguishes not-found from real I/O errors
- Digest integrity verification in both storeAdapter and validatingTarget
- Size cap on storeAdapter.Push (defense-in-depth)
- Comment clarifying double-tag behavior in Pull

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JAORMX