chore(release): format the generated CHANGELOGs

[baseballyama]

Summary

CI on main has failed its format check since the release PR (#18) was merged, because of the CHANGELOGs that changesets generated. This formats them and makes the release workflow format its own output from now on.

Motivation

changeset version puts a blank line between a bullet and its nested list. oxfmt removes that blank line. As a result:

• the release PR's CI failed (run 37405698676);
• after the merge, CI on main failed on CHANGELOG.md and packages/editor/CHANGELOG.md (run 37571744421).

Every future release PR would fail the same way. There is no linked issue; the maintainer reported the failure.

Changes

• CHANGELOG.md and packages/editor/CHANGELOG.md: formatted with oxfmt. Only the blank lines before nested lists are removed; the text is unchanged.
• New version-packages script (changeset version && oxfmt). The release workflow now uses it as changesets/action's version command, so the release PR is formatted when it is opened.

No changeset: nothing user-visible changes.

Testing

• pnpm format:check reports no issues in tracked files. The only findings are local untracked .claude/worktrees files.
• The workflow change takes effect on the next release PR. It runs the same oxfmt command that CI checks with.

Breaking changes

None

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a breaking change, I have added a changeset / CHANGELOG entry and flagged it above.
• I have re-read my own diff and removed dead code, debug prints, and stale comments.
• If I used an LLM to draft this PR, I have verified each change myself, the PR represents real work that warrants a maintainer's review, and I am willing to defend each line in review.

🤖 Generated with Claude Code

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