docs: AGENTS.md §13 system-anatomy canon pointer
Phase A of the system-anatomy canon shipped in the extro repo
at cmo/brand/system-anatomy.md.
Add AGENTS.md §13 to direct in-repo coding agents writing
owner-facing strings to that canon for UI copy, settings,
README/INSTALL prose, error messages, and onboarding.
Keep internal architecture vocabulary unchanged: observe/,
the capture pipeline, and com.solstone.capture stay as-is;
capture remains a code-only word.