[Feature] Support Element Screenshot

[CupNoodleFork]

I would like to request support for taking screenshots of specific elements in agent-browser, similar to Playwright’s element screenshot capability.

Playwright already provides an element screenshot feature, which allows capturing a screenshot of a specific DOM element:
https://playwright.dev/docs/screenshots#element-screenshot

It would be great if agent-browser screenshot could also support element-level screenshots.

For example, a possible API design could be:

agent-browser screenshot --selector @e1
agent-browser screenshot --selector ".header"

This would allow users to capture screenshots of specific elements instead of the full page.

Thank you for considering this feature request!

[CupNoodleFork]

I later noticed that there is already logic related to selector in the codebase:

agent-browser/cli/src/commands.rs

Line 445 in d97e201

// screenshot [selector] [path]

However, this does not appear to be mentioned in the documentation. and also not be mentioned in agent-browser screenshot --help. XD

It looks like this feature may already be implemented, but the documentation has not been updated accordingly.

Thank you!