snapshot uploaded every frame to Gemini whenever GEMINI_API_KEY or
GOOGLE_API_KEY was set, even with no flag. Frames now stay local unless
--describe is passed. A bare --describe uses the default prompt (it used
to send an empty one), --describe "question" asks a custom one, and
--describe false still works.
Fixes#6138
Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>