install: symlink global AGENTS.md (auto-load pi-extensions skill at session start)

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.

AGENTS.md

@@ -19,6 +19,7 @@ This file is for agents modifying the repo.
 install.sh              # Idempotent installer (symlink + cp + probes)
 pi-env.zsh              # Shell loader sourcing ~/.config/pi/.env (POSIX-compatible)
 keybindings.json        # ~/.pi/agent/keybindings.json — mosh/tmux newline fix
+pi-global-AGENTS.md     # ~/.pi/agent/AGENTS.md (symlink) — global agent instructions
 settings.example.json   # Template for ~/.pi/agent/settings.json (copy + edit)
 README.md               # User-facing quickstart + new-machine deploy recipe.
 AGENTS.md               # This file.