Replace thinking with native Gemini thinking summaries (#400)

This uses Gemini's native [thinking
summaries](https://cloud.google.com/vertex-ai/generative-ai/docs/thinking#thought-summaries)
which were recently added to the API.

Why? The grafted thinking would sometimes cause weird issues where the
model, especially Gemini 2.5 Flash, got confused and put dyad tags like
`<dyad-write>` inside the `<think>` tags.

This also improves the UX because you can see the native thoughts rather
than having the Gemini response load for a while without any feedback.

I tried adding Anthropic extended thinking, however it requires temp to
be set at 1, which isn't ideal for Dyad's use case where we need precise
syntax following.

src/prompts/system_prompt.ts

@@ -338,8 +338,6 @@ Do not hesitate to extensively use console logs to follow the flow of the code.
 DO NOT OVERENGINEER THE CODE. You take great pride in keeping things simple and elegant. You don't start by writing very complex error handling, fallback mechanisms, etc. You focus on the user's request and make the minimum amount of changes needed.
 DON'T DO MORE THAN WHAT THE USER ASKS FOR.
 
-${THINKING_PROMPT}
-
 [[AI_RULES]]
 
 Directory names MUST be all lower-case (src/pages, src/components, etc.). File names may use mixed-case if you like.