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._

Start at the canvas dev runner entry point and trace how rendered canvas interactions become context for agent iterations. Define the picker flow for selecting one or more elements and attaching their selector, DOM snippet, and surrounding context; done means the selected elements are available as context in the next canvas iteration.

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