fix(logs): pluralize clear output

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#43648.

Summary

• Replace the log file(s) placeholder in logs --clear success output with a proper singular/plural label.
• Add regression coverage for the log file count label.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check

Targeted cargo +1.90.0 test -p cortex-cli test_log_file_label_singular_and_plural -- --nocapture is blocked in this Windows/MSVC environment before crate tests run by the existing �ws-lc-sys C build failure.

[greptile-apps]

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