docs(rfc): require issues before RFCs

[drew]

Summary

Update RFC guidance so users open GitHub issues before RFCs, maintainers request RFCs only when necessary, and maintainers assign RFC numbers from the originating issue. Also correct stale RFC status metadata for existing RFCs.

Related Issue

None.

Changes

• Require new features to start from the feature request template rather than RFC PRs.
• Document maintainer-controlled RFC number assignment from issues.
• Align the RFC template, contributor guidance, architecture overview, and feature request template.
• Correct existing RFC front matter statuses: RFC 0000 to accepted, RFC 0001 and RFC 0002 to accepted, RFC 0003 to implemented.

Testing

• mise run pre-commit passes
  • Attempted, but local toolchain/dependency setup failed before completing: python:proto could not find grpc_tools on its first concurrent run, rust:lint could not build z3-sys because z3.h was missing, and aws-lc-sys failed while Homebrew GCC compiled against a macOS SDK header that uses block syntax.
• git diff --check passes
• mise run markdown:lint:md passes
• Unit tests added/updated (not applicable; docs/template-only)
• E2E tests added/updated (not applicable; docs/template-only)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)