docs: add AGENTS.md maintenance guidance

AGENTS.md

@@ -3,4 +3,11 @@
 Deliberate decisions in this repo - do NOT silently revert them:
 
 - `homebrew.onActivation.cleanup = "zap"` in `configuration.nix` is intentional. It forces the good habit of declaring every Homebrew package in the Nix config instead of installing things ad-hoc, which keeps the machine reproducible. Do not soften it to `uninstall` or `none`. Users are warned about its effect in README.md; this note is for anyone tempted to change the setting itself.
-- Never commit `.no-mistakes/` validation evidence to this public repo. `.no-mistakes/` is gitignored; if a validation pipeline stages evidence into a branch, drop it before merging.
+- Never commit `.no-mistakes/` validation evidence to this public repo. `.no-mistakes/` is gitignored; if a validation pipeline stages evidence into a branch, drop it before merging.
+
+## Maintaining this file
+
+Keep this file for knowledge useful to almost every future agent session in this project.
+Do not repeat what the codebase already shows; point to the authoritative file or command instead.
+Prefer rewriting or pruning existing entries over appending new ones.
+When updating this file, preserve this bar for all agents and keep entries concise.