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

[mhenrixon]

Summary

Adopts getzazu/app#4104 for this repo.

• Moved the whole root CLAUDE.md body into a new AGENTS.md, with a short intro explaining
  it's the single instruction file: Claude Code imports it via CLAUDE.md, and Grok, Cursor,
  Copilot and Codex read it directly. No existing AGENTS.md to merge with; no
  <!-- BEGIN docs-kit --> block in this repo. Added an adapted "Screenshots on PRs and issues"
  section, since the dashboard (app/controllers/pgbus/, app/views/pgbus/*.html.erb,
  app/frontend/pgbus/ — Tailwind + Turbo + ApexCharts) is a real UI.
• Root CLAUDE.md is now the zazu-shaped stub (@AGENTS.md + a two-line pointer). No nested
  CLAUDE.md files exist in this repo.
• Fixed the 5 places across .claude/commands/plan.md and .claude/commands/github-review-comments.md
  that pointed at CLAUDE.md meaning "the project rules" — now AGENTS.md. No
  .github/copilot-instructions.md exists. Checked pgbus.gemspec's spec.files (git-ls-files
  filtered to app/, config/, exe/, lib/ + a few root docs) — neither CLAUDE.md nor
  AGENTS.md ships in the gem, same as before.
• This repo has no .grok/ or .cursor/ directories, so the Grok/Cursor-specific hunks in the
  reference diff don't apply here.

.rtk/filters.toml: added one filter, rake-build, for bundle exec rake build (the
"Verify gem builds" step CI runs) — the global PreToolUse hook does not rewrite it. The task
sh-echoes gem-build/unpack/find/rm commands and then lists every file in the unpacked gem
(currently ~300 lines); on a real run that echo/list content is noisy for an agent (the number of
packaged files, not a violation, is the only signal). Filter: strips blank lines and the
/tmp/gem-verify/... file-listing lines, keeps the build/unpack confirmation, version/name/file
metadata and any failure output untouched.

Before → after on a real run (rtk bundle exec rake build 2>&1 | wc -l): 326 → 10 lines
(97% reduction). Verified a failing build (rake build:nonexistent) still surfaces the full
error and a non-zero exit code — the filter never touches stderr/failure output.

Considered and skipped as filter candidates (either already rewritten by the global hook, output
was already compact, or too slow to justify a line-stripping filter):

• bin/rubocop — already refused by the hook (unknown to it); AGENTS.md already says to use
  bundle exec rubocop.
• bun run lint:herb — already rewritten by the global hook (rtk bun run lint:herb).
• bin/release list — 12 lines, no noise.
• bundle exec bundle-audit check --update — 9 lines, no noise.
• bundle exec rake bench / bench:* — every line is a real benchmark number, nothing to strip
  as boilerplate, and a full run takes minutes; not "cheap" to verify and not obviously "noise".
• zazu's brakeman / lingo / rails-db filters don't apply — this repo runs none of those
  commands (it's a gem, not a Rails app; no bin/lingo, no validate_all, no db:* tasks at the
  repo root).

Added the rtk paragraph to AGENTS.md's Commands section, and an idempotent
rtk trust --yes >/dev/null 2>&1 || true step (only runs when rtk is on PATH) to bin/setup.

Test plan

• rtk trust --yes in the worktree, then rtk verify — 155/155 inline filter tests pass.
• rtk hook check "bundle exec rake build" → rewrites to rtk bundle exec rake build.
• rtk bundle exec rake build end-to-end: exit 0, output 326 → 10 lines (captured above).
• rake build:nonexistent through the same filter still prints the full Rake backtrace and
  exits 1 — confirms the filter doesn't mask failures.
• bash -n bin/setup — syntax OK. No Ruby files changed, so bundle exec rubocop doesn't
  apply; no markdown/yaml lint config in this repo.
• grep -rn "CLAUDE.md" across the repo (excluding .git, vendor, node_modules,
  gitignored .claude/worktrees/) — only the two command files above, both fixed.

---

Summary by cubic

Project instructions now live in AGENTS.md instead of CLAUDE.md, so Grok, Cursor, Copilot, and Codex read the same rules as Claude Code. Adds a project-local rtk filter for rake build so agents see gem metadata without the ~300-line file listing.

Details

• CLAUDE.md is now a stub that imports AGENTS.md; Claude Code's behavior is unchanged.
• The two .claude command files that referenced CLAUDE.md as project rules now reference AGENTS.md.
• The filter drops blank lines and /tmp/gem-verify/ listing lines and keeps the build and unpack confirmation plus gem metadata.
• bin/setup re-trusts rtk filters when rtk is available.

Side effects

• Failed builds still show the full error and exit non-zero because stderr is never filtered.
• When nothing remains after filtering, the output says so and points at the exit code instead of printing ok.
• Neither CLAUDE.md nor AGENTS.md ships in the gem.

^{Written for commit 45743f1. Summary will update on new commits.}