Move docs/writing.md to docs/AGENTS.md

The authoring-conventions page published on the website at /docs/writing; it is
guidance for contributors and agents, not site content. The website sync now
excludes AGENTS.md basenames (ProfessionalWiki/NeoWiki-Website#97), and the
AGENTS.md name makes agent tooling load the conventions automatically when
working under docs/. Content unchanged.

Merge NeoWiki-Website#97 first, so the docs rebuild this merge triggers
already excludes the file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: JeroenDeDauw