docs: remove internal TESTING.md from public catalog

TESTING.md documented an internal pre-release workflow (private-repo
clone instructions, internal ticket references) that doesn't belong
in the public agent-skills catalog and isn't part of the documented
repo layout in CLAUDE.md. Content preserved externally before removal.

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

Author: kittisakLS