Add keybindable action for file/image attachment picker

Feature Request

Add a new keybindable action (e.g. chat:addAttachment) that opens a file/image picker dialog, so users can bind it to a shortcut like Cmd+U.

Current behavior

Files and images can only be added by:

• Dragging into the terminal
• Typing @file in chat
• Pasting images with Ctrl+V (chat:imagePaste)

There is no action in the keybindings system to open a file/image browser.

Proposed behavior

• New action: chat:addAttachment (or chat:filePicker)
• Opens a native file picker dialog to select files or images to attach to the current chat
• Can be bound to any key (e.g. Cmd+U) via ~/.claude/keybindings.json

Use case

Quick keyboard-driven workflow for attaching screenshots, code files, or images without leaving the terminal flow.