Adds a testing guide covering how to verify features against a running stand:
drive the behaviour under test through the browser (not the API), verify
out-of-band in the DB/git, and the non-obvious traps. Notably the page has two
ProseMirror editors — [aria-label='Page title'] (non-collab) and
[aria-label='Page content'] (the collab body); querySelector('.ProseMirror')
returns the title, so tests must target the body editor and wait ~10s for the
hocuspocus store debounce. Links the new doc from AGENTS.md next to dev-stand.md
and adds a matching gotcha #8 to dev-stand.md.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
C9 Tester