docs: drop the org-wide claim from CONTEXT.md

[lesnik512]

Why

CONTEXT.md still says the AGENTS.md conventions "apply across all repos in the
org, not just this one". #73 deleted that exact claim from AGENTS.md itself, so the
two files now contradict each other.

The claim is false either way. AGENTS.md is not a GitHub default community health
file, so it is never inherited, and no other repo's AGENTS.md references this one.
An agent reads the file in the checkout it is standing in.

Design

Delete the sentence. Nothing replaces it: an agent reading CONTEXT.md is already in
this repo and cannot act on a statement about repos it will never see. The paragraph
above it already says what this repo is, including that it hosts the community health
files other repos do inherit.

Non-goals

• Not touching the glossary. No term changes.
• Not adding a pointer to CONTRIBUTING.md. That file is for outside human
  contributors and already states its own scope.

Verification

uv run pytest: 123 passed, 26 skipped, 5 failed. The five are PNG rendering and
reproduce unchanged on main — rsvg-convert and pngquant are not installed
locally; CI installs them. This change touches no Python and removes no link.