Add pi-global-AGENTS.md, symlinked by install.sh to ~/.pi/agent/AGENTS.md
(pi's global-instructions file, loaded at every startup). Directs the agent
to read the pi-extensions skill at session start and carries a core
fork/recall cheat-sheet, since on-demand skill description-matching was
leaving the extensions under-utilised.
Symlinked (not cp'd) like keybindings.json: pi never rewrites AGENTS.md, and
the entrypoint re-runs install.sh on every container start, so the directive
self-heals across container recreation / volume wipes.