Improved Developer tools

Internal: finished plans archived, small leftovers in TODO.md

Thirty-two finished plans and nine old briefs moved to docs/archive; their leftovers are in the TODO.md backlog.

  • TODO.md at the repository root is now the backlog of small jobs: security findings, small code fixes, documentation, small features, browser and device checks, and owner checks.
  • Plans whose areas are built moved from docs/plans/ to docs/archive/, with links updated. The root briefs for built areas moved there too, and the root ONBOARDING.md was removed.
  • docs/plans/ keeps only work still under way.