feat(sdk,cli): add ask_user tool for interactive questioning

[Harrison Chase (hwchase17)]

Add ask_user tool that allows the agent to ask the user one or more questions during execution. Uses LangGraph interrupts to pause execution and resume with user answers.

Key design decisions:

• Opt-in: Enabled via enable_ask_user=True on create_deep_agent() or --ask-user CLI flag
• Question types: Text (free-form) and multiple choice (with always-available "Other" option)
• Interrupt-based: Uses langgraph.types.interrupt() to pause the agent and collect user input, then resumes execution with the answers
• CLI modal: Renders as an inline widget in the chat (like the approval menu) with text inputs and selectable choices

Changes:

Core library (libs/deepagents):

• deepagents/middleware/ask_user.py — New AskUserMiddleware with ask_user tool, Question/Choice/AskUserRequest/AskUserResponse TypedDicts, system prompt injection
• deepagents/graph.py — Added enable_ask_user parameter to create_deep_agent()
• deepagents/__init__.py, deepagents/middleware/__init__.py — Export AskUserMiddleware

CLI (libs/cli):

• widgets/ask_user.py — New AskUserMenu Textual widget with text inputs and multiple choice selection
• textual_adapter.py — Handle ask_user interrupts (distinguished from HITL approval by type: "ask_user")
• app.py — _request_ask_user() method + on_ask_user_menu_answered/cancelled handlers
• agent.py — Pass through enable_ask_user to create_deep_agent()
• main.py — --ask-user CLI flag
• tool_display.py — Format ask_user tool calls (shows question count)
• app.tcss — Styles for the ask-user widget
• widgets/__init__.py — Export AskUserMenu