feat(compact): use small model to summarize in /compact

[timoclsn]

• Also use small model in /compact summarize feature

I think it makes sense to use the small model in summarize as there is no need for the big models there (also way faster).

amp also uses Gemini 2.5 Flash for summarization:

• https://ampcode.com/news/better-faster-cheaper-summaries
• https://x.com/thorstenball/status/1938589889567547417