fix(ai-clis): switch OpenCode AI to official standalone installer

Replace npm install with the curl-based installer that is now the
primary recommended method on opencode.ai.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pofallon

src/ai-clis/install.sh

@@ -87,7 +87,7 @@ fi
 # OpenCode AI
 if [ "${OPENCODE}" = "true" ]; then
     echo "Installing OpenCode AI..."
-    npm install -g opencode-ai
+    curl -fsSL https://opencode.ai/install | bash
 fi
 
 # CodeRabbit CLI