Add rtk clang-format filter (summarize reformatted files)

Summary

Add an rtk clang-format filter. clang-format -i is run across many files and its value to the model is essentially binary (which files changed / any errors), not the invocation echo.

Why

rtk discover flagged clang-format as an unhandled command: 52 invocations over 30 days (e.g. clang-format -i <files...>), with no RTK equivalent.

Suggested behavior

• For -i (in-place): summarize to the list of files reformatted (or "no changes"), surface any parse/error output verbatim.
• For --dry-run/--Werror: preserve the diagnostics, drop redundant per-file headers when clean.