Q&A Dialog: ask side questions without breaking the current chat flow

[kkmkoi]

Problem

When reading a long response, I often get sudden ideas or questions —
not just terminology, but related thoughts that need a quick answer.
Currently, asking interrupts the flow:

1. I ask → agent responds → the previous response scrolls away
2. Side questions pollute the conversation history
3. I lose my train of thought

Proposed Solution

A Q&A dialog for side questions without breaking the main chat:

• Highlight text (optional, for context) → press / → command palette → select "Ask a
  question"
• Dialog opens, highlighted text auto-attached
• Same session context, same model, same token tracking
• Answer appears inside the dialog
• Esc closes, main chat unchanged
• Rolling Q&A history in the dialog

Implementation Plan

I'll implement this feature first and record a demo video with code changes
to illustrate the approach:

• New file: internal/ui/dialog/qna.go — Q&A dialog
• Modified: internal/ui/dialog/commands.go — add "Ask a question" entry
• Modified: internal/ui/model/ui.go — wire command to dialog

Pure UI changes. No agent logic modifications needed.