browse: add `viewport auto` / `viewport reset` to unpin fixed viewport

[markddyer]

Problem

browse viewport W×H pins the viewport via Playwright's setViewportSize, overriding the launch-time viewport: null that normally lets the viewport follow the real Chrome window size. Once pinned, there is no way to restore auto-follow behaviour without browse restart, which kills the Chrome session (and drops a /connect-chrome headed attachment back to launched mode).

Several skills call $B viewport as a side-effect (e.g. responsive testing, benchmarks with specific breakpoints), leaving the browser in a locked state even after the skill finishes. From the user's perspective the Chrome window is visibly larger than the viewport Claude is rendering into.

Repro

$B viewport 1280x520
$B js "window.innerWidth + 'x' + window.innerHeight"   # 1280x520
# resize Chrome window manually → viewport stays 1280x520

Proposal

A browse viewport auto (or viewport reset / viewport unpin) command that clears the pinned size and restores window-following behaviour, without tearing down the Chrome session.

In Playwright terms this means calling something like page.setViewportSize(null) where supported, or recreating the page/context with viewport: null. If neither is possible without a restart, even documenting the constraint + exposing a lightweight "soft restart that preserves the current tab/URL" would help.

Environment

• Platform: Windows 11
• Mode: both headed (via /connect-chrome) and launched reproduce

[jbetala7]

Opened #1881 to add browse viewport auto (aliases reset/unpin). It rebuilds the context with viewport: null via the existing viewport --scale recreate path, so cookies/storage/tab URLs survive and the viewport follows the window again without a browse restart. Refused in headed mode (where the viewport already tracks the real window), and deviceScaleFactor resets to 1 since Playwright forbids a custom scale with a null viewport.