feat(agent): add human-in-the-loop interrupt system

[jsamuel1]

Description

Port the interrupt system from the Python SDK to TypeScript, enabling human-in-the-loop workflows where hook callbacks can pause agent execution to request human input.

Changes

• Interrupt class and InterruptException for pausing agent execution
• BeforeToolCallEvent.interrupt() API for hook-based interrupts
• BeforeToolCallEvent.cancelTool for tool cancellation without interrupt
• Agent.resumeFromInterrupt() for resuming after human response
• InterruptEvent stream event emitted when interrupts pause the loop
• AgentResult.interrupts and stopReason: 'interrupt' support
• _InterruptState for tracking active interrupts and resume flow
• Deferred tool execution on resume (skip model, re-execute pending tools)

Testing

• 14 unit tests for interrupt primitives
• 5 agent interrupt integration tests (interrupt, resume, cancel, stream events)
• All 1096 existing tests pass

[pgrayy]

Thank you for raising this PR but unfortunately we need a bit more time to discuss the design with the team. While the interface will likely remain the same as what we have in Python, we want to see if we can simplify the implementation.