Enable writable hooks for tool and result modification

[zastrowm]

Motivation

The TypeScript SDK currently only supports read-only hooks, preventing hooks from modifying event properties to change agent behavior. The Python SDK allows hooks to write to certain properties (via _can_write() mechanism), enabling use cases like redirecting tool calls, modifying tool parameters, and transforming tool results.

This implementation achieves parity with the Python SDK by making specific hook properties writable.

Resolves #17

Public API Changes

The following hook event properties are now writable (readonly modifiers removed):

BeforeToolCallEvent:

// Before: tool and toolUse were readonly
const hook = {
  registerCallbacks: (registry) => {
    registry.addCallback(BeforeToolCallEvent, (event) => {
      event.tool = differentTool      // Now allowed
      event.toolUse = modifiedToolUse // Now allowed
    })
  }
}

AfterToolCallEvent:

// Before: result was readonly
const hook = {
  registerCallbacks: (registry) => {
    registry.addCallback(AfterToolCallEvent, (event) => {
      event.result = modifiedResult // Now allowed
    })
  }
}

The agent reads potentially modified values after invoking hooks and uses them for tool execution and result handling. This follows the same pattern as the existing AfterModelCallEvent.retryModelCall property.

Use Cases

• Tool redirection: Change which tool gets executed based on runtime conditions
• Parameter sanitization: Modify tool inputs before execution (e.g., adding authentication, filtering sensitive data)
• Result transformation: Post-process tool results before they're sent to the model (e.g., formatting, filtering, error recovery)
• Error handling: Convert tool errors to success results or vice versa