Add element pick-and-polish tool to canvas dev runner for selecting elements as context

## Feature request

Add a "pick and polish" element-selection tool into the canvas dev runner.

### Motivation
When iterating on canvases, it's hard to point the agent at a specific part of the rendered canvas. We want a way to visually select elements directly in the canvas runner and add them as context for the next iteration.

### Proposal
- Add an element picker (à la browser devtools "inspect element") to the canvas dev runner.
- Let the user click/select one or more elements in the rendered canvas.
- Capture the selected element(s) — e.g. selector, DOM snippet, surrounding context — and add them as context when iterating on the canvas with the agent.

### Outcome
Faster, more precise canvas iteration: select an element, hand it to the agent as context, and ask for changes scoped to that element.

_Filed from Copilot CLI._