Adds a built-in, opt-in codemode tool that runs model-written JavaScript in the codemode sandbox. Scripts call the other active tools as tools.<name>(args), including concurrently with Promise.all; only the return value, console output, and images attached with image() reach the model.
Nested calls run through the agent loop via a new AgentToolContext (5th execute argument, ctx.executeTool for extension tools), so argument validation and the beforeToolCall/afterToolCall hooks apply. Hooks and tool_call/tool_result events carry the parent call id.
Tools can declare outputSchema and return structuredContent; codemode passes that to scripts and falls back to text content otherwise. The codemode package gains globals, declaration rendering from JSON Schema, and optional deadlines.
