chore(rtk): AGENTS.md as the shared instruction file

[mhenrixon]

Summary

Adopts getzazu/app#4104: AGENTS.md becomes the single project-instruction
file, read directly by Cursor, Copilot, Codex and any other AI tool; Claude
Code now reaches it through a CLAUDE.md stub (@AGENTS.md).

• Moved the full body of root CLAUDE.md (124 lines: Tech Stack, Critical
  Rules, Commands, Slash Commands, Architecture, the mental model, Model
  tiers, Testing, Release & docs deploy, More Documentation) into AGENTS.md,
  product rules first. Merged in the old AGENTS.md's two workflow sections
  ("Change the gem", "Write a docs page") — the "Change the gem" bullets
  duplicated the Critical Rules verbatim and were dropped (no rule lost); the
  docs-page workflow is unique and kept in full.
• Root CLAUDE.md is now a 6-line stub in zazu's shape (@AGENTS.md import
  • a pointer sentence). Nested docs/AGENTS.md (its own <!-- BEGIN docs-kit --> block, auto-managed by docs_kit:install) is untouched —
    the repo's root AGENTS.md has no docs-kit block of its own, so there was
    nothing to preserve byte-for-byte there.
• Fixed 7 pointers across .claude/ that meant "the project rules" and said
  CLAUDE.md: .claude/README.md (x2), .claude/SKILL_TEMPLATE.md (x1),
  .claude/commands/github-review-comments.md (x3), .claude/commands/plan.md
  (x2). No .github/copilot-instructions.md exists. The gemspec's s.files
  already ships only exe/, lib/, app/, config/, CHANGELOG.md,
  LICENSE.txt, README.md — neither CLAUDE.md nor AGENTS.md was ever
  packaged, so no change there. No .gitattributes/.npmignore in this repo.
• Added a "Screenshots on PRs and issues" section to AGENTS.md (forms are
  visual: rendered Forms::/PhlexForms:: markup in both themes, and the
  docs/ site), trimmed to this repo's terms — capture with
  agent-browser screenshot against docs/ (bin/dev) or a spec-rendered
  page.
• No .rtk/filters.toml added. Every verification command this repo
  documents and actually runs (bundle exec rspec, bundle exec rubocop lib spec / bundle exec rubocop, bin/rspec) is already rewritten by
  the global PreToolUse hook (checked with rtk hook check). bin/release
  already produces hand-curated, low-noise output (colored step markers) and
  has release/push side effects, so it wasn't run. docs/bin/ci and
  docs/bin/brakeman exist but aren't part of any documented workflow here
  (root CI only runs rubocop + rspec; brakeman only shows up inside the
  docs Rails app's own default CI script, which nothing in AGENTS.md,
  the Rakefile, or README.md tells an agent to run). A fake filter is
  worse than none, so I added the rtk usage note to AGENTS.md's Commands
  section without the filter-file sentence, as the brief allows.

Test plan

• rtk trust --yes (no-op: no .rtk/filters.toml) + rtk verify — 154/154 rtk-internal tests pass, hook registered
• rtk hook check on bundle exec rspec, bundle exec rubocop lib spec, bin/rubocop, brakeman, bin/release, rake release[...], gem build --strict, docs/bin/ci, docs/bin/rubocop, docs/bin/brakeman to confirm what the global hook already covers before deciding no project filter was needed
• No Ruby files changed — bundle exec rubocop not run (docs/config only)
• No markdown/yaml linter configured in this repo
• git status clean after commit; docs/AGENTS.md and all other nested files untouched

---

Summary by cubic

Makes AGENTS.md the single source of project instructions for all AI tools: the full brief moves out of CLAUDE.md, which becomes a short stub that imports AGENTS.md. All .claude pointers to the project rules now reference AGENTS.md.

Migration

• Cursor, Copilot, and Codex now read the full project brief from AGENTS.md; Claude Code loads it through the stub.
• The old "Change the gem" workflow was dropped because it duplicated Critical Rules; the docs-page workflow and a new "Screenshots on PRs and issues" section are now in AGENTS.md.
• No .rtk/filters.toml was added: the global PreToolUse hook already rewrites every documented verification command.
• The nested docs/AGENTS.md is untouched.

^{Written for commit 0cc6890. Summary will update on new commits.}