This PR implements the behind-the-scenes subagent execution logic:

### Core subagent execution
- Add `Thread::new_subagent()` constructor for creating subagent threads
- Implement `SubagentTool::run()` to spawn and manage subagent lifecycle
- Add `SubagentContext` for parent-child thread relationship
- Implement `submit_user_message()`, `interrupt_for_summary()`,
`request_final_summary()`
- Add timeout support and context-low detection (25% threshold)
- Propagate cancellation from parent to child threads

### Thread management
- Add `MAX_SUBAGENT_DEPTH` (4) and `MAX_PARALLEL_SUBAGENTS` (8) limits
- Add `register/unregister_running_subagent()` for tracking
- Add `restrict_tools()` for allowed_tools filtering
- Add `is_subagent()`, `depth()`, `is_turn_complete()` accessors

### Thread changes
- Add `ToolCallContent::SubagentThread` variant
- Add `ToolCallUpdateSubagentThread` for UI updates
- Add `tool_name` field for subagent detection
- Add `is_subagent()` method on `ToolCall`
- Add image support in `ContentBlock`

Release Notes:

- N/A

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Co-authored-by: Amp <amp@ampcode.com>