Add skill/SKILL.md (+ skill/evaluate-extension-usage.py, referenced by the
skill via ./) so the canonical 'how to use fork/recall/ssh-controlmaster'
skill lives next to the extensions it documents — the single source of truth.
Motivation: the global AGENTS.md (pi-toolkit) tells every pi session to read
~/.agents/skills/pi-extensions/SKILL.md at session start to fix fork/recall
under-utilisation, but that skill previously lived ONLY in the private
skillset repo. In any environment without the skillset mounted (e.g. a
pi-devbox container started without it) the pointer dangled. Co-locating the
skill here gives a public, package-owned source that downstreams can vendor.
install.sh is intentionally unchanged: skill deployment on a normal
workstation stays the skillset repo's responsibility (no double-deploy).
Joakim Persson