feat(cli): expose model thinking events in --output-format stream-json

[KobiAms]

Problem

When running gemini -p "..." --output-format stream-json with a thinking-capable model (e.g. Gemini 2.5 Pro), all Thought events emitted by the model are silently dropped. Developers building integrations on top of the non-interactive CLI have no way to observe the model's reasoning process.

The interactive UI already handles thinking events (useGeminiStream.ts), but the non-interactive stream-json path had no corresponding support.

Proposed Solution

• Add a THINKING = 'thinking' event type to JsonStreamEventType in packages/core/src/output/types.ts
• Add a ThinkingEvent interface (content: string, subject?: string) to the stream event type union
• Handle GeminiEventType.Thought in the non-interactive response loop (nonInteractiveCli.ts) to emit structured thinking events when using --output-format stream-json

Example output (stream-json)

{"type":"thinking","timestamp":"...","content":"**Analysis** Let me think through this step by step...","subject":"Analysis"}

Impact

No breaking changes. The thinking event is additive — consumers that don't handle it can ignore it. Consumers that want model reasoning now have a structured way to access it.

[SandyTao520]

Not sure if we want to output all thoughts by default. Might be a good to have for verbose mode.

[x157]

✅ Please add this feature!

If you want to lock it behind a --show-thinking or similar flag, that's fine.

Please give us the ability to see this critical info.