docs: polish Briefprint release notes

[dd3ok]

Summary

• align README and Korean README cache-dir examples under .cache/briefprint
• mark the post-v0.4.0 explicit-use policy changes as 0.5.0 - Unreleased
• record latest fetched origin/main CI success in VALIDATION.md without claiming CI for uncommitted local changes

Validation

• python -m pytest -q -p no:cacheprovider -> 169 passed, 2 skipped
• python scripts\validate_skill.py --run-evals -> OK
• ruff check . -> All checks passed
• git diff --check -> no whitespace errors; CRLF normalization warnings only
• gh run view 28759630346 -> completed/success for 677057a

[gemini-code-assist]

Code Review

This pull request updates the documentation and changelog to align with the upcoming 0.5.0 release. Specifically, it updates the cache directory paths in both the English and Korean README files to use .cache/briefprint instead of .cache, refines a Korean translation regarding secret redaction, updates the changelog, and records the latest validation and CI status. There are no review comments, and I have no feedback to provide.

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