Remove gitoxide: standardize local read-only ops on the git subprocess backend

[staging-devin-ai-integration]

Summary

Removes the gix (gitoxide) dependency and the dual-backend layer it existed for. The five local read-only operations (rev_parse, is_ancestor, for_each_ref, for_each_ref_commits, cat_file_batch_types) were previously dispatched through a LocalGitBackend trait with two implementations (in-process GixBackend — the default — and subprocess GitBackend), selected via use_gitoxide config / GIT_CACHE_USE_GITOXIDE. That meant maintaining two parity-tested implementations of every op while all mutating and network paths already went through the git binary.

- Git { ..., local_backend: Arc<dyn LocalGitBackend> }  // GixBackend | GitBackend
- Git::with_gitoxide(bool), Git::run_gix(...)
+ Git methods run the subprocess implementations directly (unchanged bodies from GitBackend)

• Deleted crates/git-cache-git/src/backend.rs and src/gix_backend.rs; the former GitBackend bodies are inlined into the corresponding Git methods, so behavior matches the previous use_gitoxide = false path exactly. Input validation (reject_revision_arg, reject_ref_arg) is unchanged and still happens before argv composition.
• Removed AppConfig.use_gitoxide, default_use_gitoxide, and GIT_CACHE_USE_GITOXIDE env parsing (unknown TOML keys and env vars were already ignored, so existing deployment configs won't break). README row dropped.
• Removed the gix/subprocess parity test in git_wrapper.rs and collapsed the _gix_backend/_git_backend duplicate regression tests in git_client_advanced.rs into one; TestServer::start_with_file_url_upstream loses its backend parameter.
• Dropped gix from the workspace and async-trait from git-cache-git (only the deleted trait used it); Cargo.lock sheds ~60 transitive gix-* crates.

Release notes

No user facing change (the previously default in-process gitoxide backend is removed; all local read-only Git operations now run through the git binary, matching the existing GIT_CACHE_USE_GITOXIDE=false behavior).

Link to Devin session: https://staging.itsdev.in/sessions/f90f17d194054a468473b35247f3177e
Requested by: @0lut

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