🤖 feat: Support attaching external files to prompts (drag-and-drop / file picker)

## Request

Attach external files (from Downloads, Desktop, etc.) to prompts without copying them into the workspace first.

**Desired UX:** Drag-and-drop or file picker (📎) in chat input.

**File types:** `.txt`, `.sql`, `.pdf`, `.docx`, images

## Current State

- `@` tagging: works for in-project files ✓
- Image paste: works ✓
- File drag-and-drop: not supported ✗

## Challenge: Context Management

Large attachments can blow context limits. Need:
- File size / token estimation before send
- Clear limits and warnings
- Strategy for PDFs (text extraction) and DOCX (parsing)

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_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking: `high`_