docs(repo): eval-informed metadata, README banner, agent guidelines

- README/marketplace/plugin descriptions reframed "battle-tested" -> "eval-informed";
  add Outcome Evals section + scientific-method result; add readme banner.
- New AGENTS.md (repo workflow: preserve 39 count, situation-named <200 desc,
  agent-native, When-NOT-to-use, don't commit backups/datasets/logs) and
  CLAUDE.md project notes.
- .gitignore: exclude backups/ (local global-Claude-state archives).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>