fix(cli): warn on native Git drift

[freezscholte]

Summary

• add a best-effort native-backend warning when Forge changed_paths includes paths that are clean in Git
• surface the warning from both save and propose so stale native-base drift is visible before accept
• add a regression covering a Git-only commit plus an actual attempt edit

Verification

• rtk cargo fmt --all --check
• rtk cargo test --workspace
• rtk cargo clippy --workspace --all-targets -- -D warnings
• rtk bash scripts/check-rust-line-count.sh
• rtk bash scripts/e2e-eval.sh
• real-binary smoke: native save reports README.md + git_only.txt and warns only for git_only.txt

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