feat(installer): add script to install pi coding agent

Adds a shell script that installs the @earendil-works/pi-coding-agent
and pi-acp npm packages globally if the `pi` command is not found.
Uses paru to install Node.js and npm on Arch-based systems when needed.

installer/pi-agent.sh

@@ -0,0 +1,9 @@
+#!/bin/sh -e
+
+if [[ ! -x "$(command -v pi)" ]]; then
+    node -v || paru -Sy --noconfirm nodejs npm
+    npm install -g --ignore-scripts @earendil-works/pi-coding-agent
+    npm install -g pi-acp
+fi
+
+