[Feature Request] Allow users to disable automatic injection of conversation history summaries

### Description of the feature request:

## Description

Currently, Antigravity automatically injects conversation history summaries (`conversation_summaries`) into every new conversation. While this feature can be helpful in some scenarios, it may not always be desirable.

I would like the ability to **opt out of or disable** the automatic injection of previous conversation summaries.

## Motivation

- **Privacy & Context Isolation**: I prefer each conversation to be treated as a fresh, independent session without any carry-over from previous interactions.
- **Reduced Noise**: The injected summaries can introduce irrelevant context, especially when switching between unrelated tasks or projects.
- **Prompt Budget**: Conversation summaries consume part of the context window, which could otherwise be used for the actual task at hand.
- **User Control**: Users should have granular control over what contextual information is provided to the assistant.

## Current Workaround

The only available workaround is adding a custom user rule to instruct the assistant to ignore the injected summaries:

> "Fully ignore all automatically injected conversation history and summaries."

However, this is suboptimal because:
1. The summaries are still injected and consume context window tokens.
2. It relies on the model's compliance rather than a system-level guarantee.
3. It requires users to manually configure a rule for something that should be a first-class setting.

## Proposed Solution

Provide a user-facing setting (e.g., in settings/preferences or `.gemini/settings.json`) to disable the automatic injection of conversation summaries. For example:

```json
{
"conversationHistory": {
"injectSummaries": false
}
}
```

Alternatively, a toggle in the UI settings panel would also work.

## Additional Context

This request is about giving users more control over their workflow. The feature itself (conversation summaries) is valuable — the ask is simply for an **opt-out mechanism**.

### What problem are you trying to solve with this feature?

_No response_

### Any other information you'd like to share?

_No response_