AGENTS.md: internal network pattern, ChangeThisPassword secrets convention,
_notes exemption, and a Known exceptions list (pihole, zammad-init,
compose-aio, VOL_PROJECTS nesting, mailu env_file, headscale config vars).
README.md: fix Preferred typo.
_docs/lint.py: checks every convention plus compose<->sample.env sync;
run with python3 _docs/lint.py.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
