docs: add CHANGELOG.md for v0.6.0 release

[teeler]

Summary

• Adds a CHANGELOG.md (Keep a Changelog format) documenting the v0.6.0 release.
• The v0.6.0 tag has been pushed and points at this commit (atop the fix: render text/html display_data output instead of raw IPython repr #58 fix on main).

v0.6.0 contents

• Added — Rich display_data rendering via render_display_data() (fix: render text/html display_data output instead of raw IPython repr #58)
• Changed — remote copy-paste OAuth2 flow (feat(auth): use remote copy-paste OAuth2 flow instead of localhost server #54)
• Fixed — keep-alive switched to TFE tunnel ping (Session becomes unresponsive after 80 seconds #14, fix(keep-alive): use TFE tunnel ping instead of RuntimeService RPC (#14) #61)
• Removed — dead grpc-web client-registry / API-key code (fix(keep-alive): use TFE tunnel ping instead of RuntimeService RPC (#14) #61)

Notes

• Version is VCS-derived via hatch-vcs, so the tag drives the package version; no pyproject.toml edit needed.

[sethtroisi]

I'd love if we wrote the changelog and then tagged it as the new version.