git.binary configuration is ignored #81

Summary

The git.binary configuration value is not used when spawning Git subprocesses.

Current behavior

The configuration exposes a custom Git binary path, but runtime code invokes Command::new("git") directly throughout the Git integration. Deployments that configure a pinned or non-default Git executable still use whichever git appears on PATH.

Expected behavior

Every Git subprocess should use the configured binary, or the unsupported configuration field should be removed.

Suggested implementation

Centralize Git command construction around the configured binary and migrate all subprocess call sites to it. Add a test using a wrapper executable to prove the configured path is honored.

Relevant code

• crates/walgit-config/src/lib.rs:569
• crates/walgit-git/src/lib.rs:615
• crates/walgit-git/src/lib.rs:2642