docs: establish Requests for Discussion process

[darwin67]

Summary

• establish an Oxide-inspired Requests for Discussion process using numbered AsciiDoc sources under rfd/NNNN/README.adoc
• add six RFDs covering administration, self-hosting documentation, API and discovery, hosted safety, workspace collaboration, and advanced developer workflows
• track implementation separately in cross-linked Org checklists while allowing either Org or Markdown checklists in the validation tooling
• define RFD metadata and lifecycle states, plus a portable status checker with fixture-based regression coverage and CI integration

Validation

• make check-rfds
• mix precommit (454 tests)
• full S3-backed test suite (453 tests)
• shellcheck scripts/check-rfd-status.sh scripts/check-rfd-status-test.sh
• docker compose config --quiet
• nix flake check --no-build