chore: bump version to 6.4.1
v6.4.0 was never shipped; 6.4.1 is the first release with these changes.
Points the OpenCode V2 pinning docs at v6.4.1, since no v6.4.0 tag will
exist. Excludes the gitignored evals/ clone from the version audit, which
was grepping all 11G of it and never finishing.
Claude-Session: https://claude.ai/code/session_01BJAzd3A26a2XKo1JUJWySu