feat: support git -C <path> flag for cross-directory operations

[pcadet]

Summary

• Add -C <path> flag support to rtk git commands, enabling cross-directory git operations
• Create git_command() helper that prepends -C <path> to all git invocations
• Thread git_dir parameter through all 12 run_* functions in git.rs
• Update hook to extract and preserve -C flags before subcommand matching via _rtk_git_rewrite() helper

Test plan

• cargo fmt && cargo clippy && cargo test — 362/362 pass, no new warnings
• Unit tests: test_git_command_with_dir and test_git_command_without_dir
• Hook test: git -C /tmp/repo status → rtk git -C /tmp/repo status
• Hook test: git -C /other/repo log --oneline -5 → rtk git -C /other/repo log --oneline -5
• Hook regression: git status → rtk git status (unchanged)
• Hook regression: git diff --cached → rtk git diff --cached (unchanged)
• Hook regression: cargo test → rtk cargo test (non-git unaffected)
• Manual: rtk git -C /tmp status → "Not a git repository" (correct)
• Manual: rtk git -C <real-repo> status → filtered status output
• Manual: rtk git status / rtk git log → unchanged behavior (no regression)

[pszymkowiak]

Thanks for this clean PR! The approach is solid — git_command() helper + threading git_dir through all run_* functions is the right pattern.

A few notes:

Code looks good:

• The Clap -C flag integration in main.rs is correct
• The hook shell helper _rtk_git_rewrite() properly extracts and preserves -C flags
• Unit tests cover git_command() with and without -C

Minor requests before merge:

1. Please remove the CHANGELOG.md and CLAUDE.md changes — these are auto-managed by release-please and maintained by the project. Only src/, hooks/, and README.md changes should be in the PR.
2. Could you add a quick integration test (even #[ignore]) that runs rtk git -C /tmp status against a real temp repo? This would catch edge cases like paths with spaces.

Question: Does this handle -C with no space (-C/path) or only -C /path? Git accepts both forms.

Overall this is mergeable with the doc changes removed. Nice work!