Hand off work to a new thread with preserved context.

Use this tool when:
- The current conversation is getting too long and context is degrading
- You need to start a focused subtask while preserving important context
- The context window is near capacity

The handoff tool uses Gemini 2.5 Flash to analyze the current conversation and extract:
- Goal and progress summary
- Files modified and current state
- Next steps and key context

A new session is created with this summarized context, allowing seamless task continuation.

Parameters:
- goal: Brief description of what should continue (required)
- follow: If true, switch to new thread immediately (default: true)
- include_files: Optional list of important file paths

Example:
```json
{
  "goal": "Continue implementing the authentication middleware",
  "follow": true,
  "include_files": ["src/auth/middleware.py", "src/auth/jwt.py"]
}
```
