fix(nixos): deploy Pi extensions globally

configs/nixos/AGENTS.md

@@ -32,6 +32,8 @@
 
 - Use produced literals for Herdr key names that include shifted punctuation.
   For example, use `prefix+<` instead of `prefix+shift+comma`.
+- Put global Pi extension sources in `configs/nixos/modules/shell/pi/extensions/`.
+  Home Manager deploys them to `~/.pi/agent/extensions/`.
 - Treat Home Manager-managed Pi extension, prompt, and skill directories as generated output.
   Do not edit those directories except for throwaway development or local experiments.
   Promote finished work into the NixOS configuration before treating it as deployed.