Agent Skills: update-changelog

Update `CHANGELOG.md` (or `CHANGELOG`) with notable user-facing changes since the last release tag. Use when the user asks to "update the changelog", "update CHANGELOG.md", "write release notes", "prepare release notes", or "add unreleased entries". Adds entries to the `## Unreleased` section, preserves existing format, and filters out trivial commits (typo fixes, internal refactors, dependency bumps without security impact).