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

[mhenrixon]

Summary

Adopts getzazu/app#4104 for this repo.

• AGENTS.md is now the single instruction file. Moved the whole root CLAUDE.md body (131
  lines) into a new AGENTS.md, with a short intro explaining it's shared by Claude Code, Grok,
  Cursor, Copilot and Codex. Root CLAUDE.md is now a 4-line stub that @-imports AGENTS.md
  (same shape as zazu's). No content was dropped or duplicated. docs/AGENTS.md (the docs
  authoring contract) is a separate, unrelated file and was left untouched, as were .claude/rules/
  and .claude/commands/.
• Fixed 4 stale CLAUDE.md pointers in .claude/commands/{lfg,plan,github-review-pr,github-review-comments}.md
  that meant "the project rules" — they now say AGENTS.md. Repo-wide grep confirms no other
  CLAUDE.md references remain outside the stub itself. importmap-plus.gemspec's spec.files
  list never included either file, so neither ships in the published gem — nothing to fix there.
  No .github/copilot-instructions.md exists in this repo.
• .rtk/filters.toml — one filter, scoped to docs/. cd docs && bundle exec rake lint is
  the only command in the verification workflow the global rtk hook doesn't already rewrite
  (rspec, rubocop and rake test are already covered). The rake lint task sh-echoes the
  full 53-file rubocop invocation as one very long line, and on failure also dumps a rake/bundler
  backtrace through Bundler::CLI/Thor internals — neither is useful signal. New filter
  docs-lint strips both; verified against real captured output for a clean pass and for a real
  RuboCop offense (Layout/TrailingEmptyLines), both rtk trust --yes + rtk verify clean
  (156/156 inline tests, including this filter's 2).
  • Line counts (real captured output, not estimates): clean pass 5 → 3 lines; offense/failure
    30 → 7 lines (plus a rtk recall pointer). The bigger win is the ~950-character echoed
    file-list line disappearing from both cases.
  • Filter placement note: the file lives at docs/.rtk/filters.toml, not the repo root. rtk
    resolves project-local filters against the directory a command is actually invoked from, with
    no upward walk — and rake lint only runs with docs/ as the working directory (it needs
    docs/Gemfile and docs/Rakefile). A root-level .rtk/filters.toml matched in rtk hook check but silently failed to apply end-to-end once the shell actually cd'd into docs/
    first; moving the file to docs/.rtk/ fixed it. Documented this in AGENTS.md so the next
    person doesn't rediscover it.
  • Added an idempotent rtk trust --yes step to docs/bin/setup (the only bin/setup in this
    repo — there's none at the gem root) — it no-ops when rtk isn't installed.
  • Did not add filters for bundle exec ruby -Itest test/*_test.rb or bin/release --dry-run: ran both for real, output is already a handful of clean lines. Did not run
    bundle exec appraisal generate (10 gemfiles, several against rails/rails branches over
    git/bundle install — not cheap) or the live-CDN bin/importmap commands, so no filter was
    built or claimed for those.
• Screenshots section: added, scoped to docs/. The gem's CLI itself has no visual output,
  but docs/ is a real deployed Phlex/docs-kit Rails site — I verified this by reading its
  app/views, app/components, app/assets structure before deciding. Added a trimmed
  "Screenshots on PRs and issues" section to AGENTS.md covering docs/ UI changes: the four
  gh … --attach examples, the quoting rule, create-vs-comment, agent-browser screenshot against
  cd docs && bin/dev, save under the scratchpad, brew upgrade gh. Dropped the native/simulator
  and bin/claude-setup mentions from zazu's version (neither applies here).

Test plan

• rtk trust --yes + rtk verify from docs/ — 156/156 inline tests pass
• rtk hook check "bundle exec rake lint" (from docs/) rewrites to rtk bundle exec rake lint
• rtk bundle exec rake lint end-to-end, clean tree — condensed output, exit 0
• rtk bundle exec rake lint end-to-end, with an injected Layout/TrailingEmptyLines offense
  — condensed output, exit 1 preserved, offense reverted before commit
• bundle exec ruby -Itest test/importmap_test.rb — 45 runs, 0 failures (gem root, unaffected
  by this PR)
• ruby -c docs/bin/setup — syntax OK
• Repo-wide grep -rn "CLAUDE.md" (outside .git) shows only the intentional @AGENTS.md
  stub reference
• git diff --stat reviewed — only the intended 8 files touched, docs/Gemfile.lock reverted
  after an unrelated local bundle install diff
• Did not run bundle exec rake test (full suite, live CDNs) or bundle exec appraisal generate — docs/config-only change, out of scope per this repo's own "don't run full test
  suites for docs/config PRs" guidance

---

Summary by cubic

Makes AGENTS.md the shared project instruction file and condenses docs lint output in agent runs.

What changed

• Moved the full instruction body from root CLAUDE.md into AGENTS.md; CLAUDE.md is now a stub that imports it.
• AGENTS.md is read directly by Grok, Cursor, Copilot, and Codex; Claude Code gets the same file through the stub.
• Updated four .claude/commands/*.md files that referenced CLAUDE.md to reference AGENTS.md.
• Added a screenshots requirement to AGENTS.md: PRs that change docs/ UI must attach before/after captures.
• Added docs/.rtk/filters.toml for bundle exec rake lint and made docs/bin/setup trust it idempotently when rtk is installed.
• The filter's empty-output message now points at the exit code instead of printing ok when a crash writes only to stderr.

Side effects

• No content was dropped or duplicated; docs/AGENTS.md remains the separate docs authoring contract.
• No gem code, dependencies, or release behavior changed.
• The filter lives under docs/.rtk, not the repo root, because rtk resolves filters only in the directory a command runs from.
• Clean lint output drops from 5 to 3 lines; failure output drops from 30 to 7, and both lose the ~950-character echoed file list.
• Test seed output is intentionally left unfiltered so order-dependent failures can be reproduced.

^{Written for commit e7b81e0. Summary will update on new commits.}