feat: support git -C directory option

[PLNech]

Summary

• rtk git -C /path/to/repo log --oneline now works (was: unexpected argument '-C')
• Adds -C flag to the Git command level, threads through to all git invocations via git_cmd() helper
• Uses thread_local! to avoid threading directory through ~30 internal functions

Test plan

• 3 new TDD tests: -C parsed, absent = None, passthrough subcommands
• All 417 tests pass
• cargo fmt && cargo clippy -- -D warnings clean
• Manual: rtk git -C /tmp/some-repo status works