feat: add Gemini engine smoke test workflow (#2171)

Add smoke-gemini workflow based on smoke-copilot pattern to validate
the Gemini engine. Tests GitHub MCP connectivity, github.com
reachability, file write/read, and safe-outputs invocation.

Requires GEMINI_API_KEY secret to be configured in the repository.

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

Author: lpcox