Prevent :@@= loads from (falsely) indicating to the server they have missing commit ancestry

[nishu-builder]

The problem:

1. :@@= shallow-fetches commit C. Git writes C’s hash into .git/shallow.
2. CAOS extracts C’s tree and includes it in a request tree R. R does not need to reference commit C.
3. Later, CAOS runs git push to upload R. Git sends roughly:

   shallow <C's hash>
   <command to update refs/caos/req/R>
   <pack containing R and the objects it needs>

Git announces the local shallow boundaries even when those commits aren’t part of what’s being pushed. Our server rejects the request as soon as it sees shallow C. It doesn’t first establish whether C is relevant to this upload.

This PR keeps the cleanup on the client. It shallow-fetches into a temporary bare repository, copies only the snapshot’s trees and blobs into the client’s object store, then deletes the temporary repository, including the fetched commit and .git/shallow. The server continues rejecting shallow pushes; it does not strip anything from the request.

A ref under refs/caos/locator-trees/<commit> remembers the tree hash and keeps the snapshot available for reuse. import_source continues to import full commit history.