feat: OAuth/PKCE write-back to actor.rpg.stats

[ibloud]

Summary

Implement Phase 2 canonical write-back to AT Proto actor.rpg.stats using OAuth/PKCE.

Implementation

• Host client-metadata.json on GitHub Pages.
• Integrate @atproto/oauth-client-browser (CDN acceptable for prototype).
• Implement fetch-merge-put to avoid overwriting unrelated actor fields.
• Update playerProfiles.did migration path.
• Test with throwaway account first.

Acceptance Criteria

• Post-game stats appear in player AT Proto record.
• Merge-safe updates do not clobber unrelated data.
• Documented token/env rotation process for production hardening.