Fixes #2323.

Summary

• Build the Gemini hook command through a small platform helper.
• On Windows, prefix the generated .sh hook with Git for Windows bash.exe when it can be found, quoting paths with spaces.
• Keep non-Windows behavior as the existing bare hook script path.

Verification

• cargo +1.93.0 test gemini_hook_command_windows -- --nocapture
• cargo +1.93.0 test gemini -- --nocapture
• cargo +1.93.0 fmt --all -- --check
• cargo +1.93.0 clippy --all-targets
• cargo +1.93.0 test --all -- --skip small_grep_not_worse_than_plain
• git diff --check