docs: AGENTS.md for every agent; CLAUDE.md imports it

[foxnne]

What changes

Other coding agents can now read the repo's instructions.

• Content moves to AGENTS.md. The instructions in the root and in sdk/, app/, core/, backend/, plugins/, tests/ and web/ now live in AGENTS.md, the file Codex, Cursor and other agents read.
• Each directory keeps a one-line CLAUDE.md stub. It reads @AGENTS.md. The root stub imports CONTRIBUTING.md too, since other agents don't follow @ imports and the root AGENTS.md now tells them to read it.
• Why stubs rather than relying on Claude Code's own AGENTS.md support: since 2.1.277 Claude Code reads AGENTS.md, but only when no CLAUDE.md exists in the directory chain. That support also depends on Claude Code's feature flags: on Bedrock, Vertex or Foundry, or with telemetry off, it isn't there. A stub that imports the file loads it every time, while the content stays in one place. The root AGENTS.md says to edit the AGENTS.md and leave the stubs alone.
• References follow the move:
  • CONTRIBUTING.md;
  • docs/PITFALLS.md;
  • the area files' cross-references;
  • code comments that said "see CLAUDE.md" (build.zig, build.zig.zon, sdk/, app/layout/, src/editor/Editor.zig);
  • bench-markdown, which embedded the root CLAUDE.md as its medium sample and would otherwise have measured a two-line stub.

core/fuzzy.zig's tests keep the string "CLAUDE.md". It's a test input for case-folding, not a reference to the file.

SDK impact

• None. sdk/AGENTS.md and sdk/CLAUDE.md aren't in sdk/build.zig.zon's .paths. core/ is vendored whole into the tarball, stub included, and the stub resolves there too.

Verified

• scripts/check-fmt.sh passes.
• zig build -l configures.
• zig build bench-markdown runs on the new sample ("medium (AGENTS.md), 19106 bytes").
• A grep finds no CLAUDE.md reference left outside the stubs, the root AGENTS.md's note about them, and core/fuzzy.zig's test strings.

🤖 Generated with Claude Code