Fix isomorphic-git internals imports

[adamziel]

What it does

Replaces the storage sparse checkout implementation with public isomorphic-git APIs for ref discovery, pack indexing, and object reads. Removes the private isomorphic-git/src/* imports, deletes the declaration shim, and avoids vendoring isomorphic-git internals.

Rationale

The previous approach depended on private isomorphic-git internals and copied a large internal implementation into the storage package. Using the supported public APIs keeps this code lower-maintenance while preserving the sparse checkout behavior needed by Playground.

Implementation

• Uses listServerRefs, init, indexPack, and readObject from isomorphic-git.
• Keeps small local pkt-line/upload-pack helpers for the partial clone object requests.
• Keeps a small direct .git/index writer in git-create-dotgit-directory.ts so it no longer imports private GitIndex.
• Preserves withObjects: true output for .git consumers: packfiles, objects, and fileOids.
• Preserves GitAuthenticationError mapping for 401 and 403 responses.

Testing

Ran:

npm exec nx test playground-storage
npm exec nx lint playground-storage
npm exec nx build playground-storage