skills: vendor mattpocock skills for the tc sandbox

Add the 22 engineering + productivity skills from mattpocock/skills under
experimental_skills/, and extend skills/install.sh to symlink them into the
tc sandbox's ~/.claude-test/skills only — never the regular ~/.claude or
Codex install. Keeps the trial set isolated from everyday skills.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>