chore: remove spike package and references (#29)
Remove the spike package which was used for early PDS testing but is no longer needed. Updates all documentation references in CLAUDE.md, README.md, environment files, and planning docs.
Changes:
- Delete packages/spike directory
- Update CLAUDE.md: remove spike from packages table and commands
- Update README.md: remove spike from packages table
- Update .env.example and .env.production.example
- Update docs: atproto-forum-plan.md, oauth-implementation-summary.md, write-endpoints-design.md
- Update pnpm-lock.yaml via pnpm install