feat: use latest mcpg container image in smoke-copilot

Set sandbox.mcp.version to 'latest' so the smoke test always exercises
the most recent published mcpg image rather than a pinned release.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox

.github/workflows/smoke-copilot.md

@@ -51,6 +51,7 @@ steps:
 sandbox:
   mcp:
     container: "ghcr.io/github/gh-aw-mcpg"
+    version: "latest"
 safe-outputs:
     threat-detection:
         enabled: false