Document the gate in AGENTS.md

Version 0.1.3 Updated Sep 27, 2026

Available from 0.1.3. Coding agents read a repository's AGENTS.md, so each consumer documents the gate there with one canonical block: the text in AGENTS.md spelling block, copied verbatim with its typos-config-builder:agents-md start and end markers. The block replaces any other spelling or Typos prose in AGENTS.md. It tells an agent to commit the typos.toml the gate regenerates, and to put a regeneration unrelated to its work in a separate base pull request. Take the block from the release the repository pins.