fix(ci): use npm package for copilot CLI instead of curl installer

The curl installer was producing corrupted binaries. npm install
@github/copilot is more reliable and version-pinnable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: christso

.github/workflows/evals.yml

@@ -32,7 +32,7 @@ jobs:
         run: bun run build
 
       - name: Install GitHub Copilot CLI
-        run: curl -fsSL https://gh.io/copilot-install | bash
+        run: npm install -g @github/copilot
 
       - name: Install Pi CLI
         run: npm install -g @mariozechner/pi-coding-agent || echo "pi-cli install failed (non-fatal)"