[Initiative] Migrate gitstore-git-service from git2 to gitoxide

[juliuskrah]

Summary

Migrate gitstore-git-service from the git2 crate (libgit2-backed) to gitoxide so core repository operations are implemented in pure Rust. This reduces the native dependency surface, aligns with the long-term plan to remove shell-outs to the git binary, and makes deeper in-process receive-pack integration more practical.

Scope

In Scope

• Replace git2 usage in gitstore-git-service with gitoxide equivalents for repository open/init, ref discovery, commit/tag inspection, and repository metadata operations.
• Update internal abstractions so repository access is no longer tied to libgit2-specific types.
• Update build configuration and dependencies to remove git2 and any libgit2-specific build requirements.
• Preserve existing HTTP smart protocol and websocket behaviour during the migration.
• Add or update tests to validate parity for repository initialization, ref discovery, and tag event handling.

Out of Scope

• Removing shell-out calls to the git binary (tracked separately in [Initiative] Remove git binary shell-outs from gitstore-git-service #109).
• Re-implementing smart HTTP packet handling in-process in this issue.
• gRPC contract design itself (tracked by [Initiative] Decouple API from Git Storage via gRPC Git Service #65).

Acceptance Criteria

• gitstore-git-service no longer depends on the git2 crate.
• Core repository operations currently backed by libgit2 are implemented using gitoxide or internal abstractions built on it.
• Existing repository initialization, ref listing, and tag/event tests continue to pass after migration.
• Build and runtime images no longer require libgit2-specific toolchain setup.
• Documentation or developer notes identify any intentionally deferred gaps between git2 and gitoxide behaviour.

Dependencies

• Supports: [Initiative] Decouple API from Git Storage via gRPC Git Service #65 ([Initiative] Decouple API from Git Storage via gRPC Git Service)

Tracking

• Milestone: TBD

[juliuskrah]

Cleanup note: this initiative still says to 'preserve existing HTTP smart protocol and websocket behaviour'. WebSocket support is being removed, so that scope should be rewritten to align with the gRPC notification stream in #139 and the no-WebSocket architecture direction.