`filter_ruff_format` matches pre-0.12 ruff wording, so `rtk ruff format --check` compresses nothing

filter_ruff_format keys off two strings:

if lower.contains("would reformat:") { … }
if lower.contains("left unchanged") { … }

ruff stopped emitting both in check mode. On ruff 0.16.6 format --check prints unformatted: File would be reformatted per file and N files already formatted as the summary, so neither branch fires and the filter falls through to result.push_str(output.trim()) — an identity function over the full ANSI diagnostic block.

Reproduction (ruff 0.16.6, LC_ALL=C, 2 files, 1 unformatted)

$ ruff format --check . | wc -c        # 886
$ rtk ruff format --check . | wc -c    # 886   -> 0.0% savings

Against CONTRIBUTING.md's 20% floor, the format path currently delivers nothing. ruff format (write mode) still says 1 file left unchanged and is still summarised correctly, so only the --check path is dead.

Verified identical on develop (665b2720) and on #3083's head — this predates that PR and is out of its scope.

Suggested direction

Match the current wording as well: unformatted: for the per-file lines and already formatted alongside left unchanged for the summary. Keeping both spellings covers old and new ruff. The existing test_filter_ruff_format_needs_formatting / test_filter_ruff_format_all_formatted fixtures use the pre-0.12 strings and pass, which is why the drift went unnoticed — new fixtures should be captured from a current ruff.

Locate the filter_ruff_format implementation and read test_filter_ruff_format_needs_formatting and test_filter_ruff_format_all_formatted first. Add coverage for current Ruff 0.16.6 check-mode output while preserving the existing wording fixtures, then run the filter tests and confirm rtk ruff format --check compresses output again.