feat(gt): add Graphite CLI support

[charlesvien]

Many developers including myself use gt as their primary git interface. Since gt passes unknown subcommands to git, gt status = git status, gt diff = git diff, etc. Without RTK support, all git output is unfiltered.

Happy to take feedback or make changes!

Summary

• Add rtk gt command with filters for log, log short, log long, submit, sync, restack, create and branch
• Route git-passthrough commands (gt status, gt diff, etc.) through RTK's existing git filters for free token savings
• Unknown subcommands fall through to gt directly via external_subcommand
• Translated the remaining French doc comments in utils.rs to English

How did I test this?

• cargo fmt && cargo clippy --all-targets && cargo test --all (440 pass)
• All 26 gt_cmd tests pass including exact-format regression tests and 60%+ token savings assertions for log, submit, sync, create and restack filters
• Manual: rtk gt log short, rtk gt log, rtk gt branch info
• Manual: rtk gt status (verify git passthrough routing)