Limit figure width to CSS size

Captures are taken at 2× device scale, and the docs site stretches every figure to the content column. A narrow subject (the Worktrees slot details drawer, 460 CSS px, stored at 920×1800) therefore renders at about twice its real size, so one panel fills several screens and its text looks enlarged. Requirement: render each figure no wider than its CSS-pixel width (its PNG width ÷ 2), still capped at the column width, e.g. by having the build or a remark plugin set the intrinsic width from the PNG. Out of scope: retaking captures; changing the capture helper's scale. Done means: on the built site, `console-reference/automation/worktrees` shows the details figure at about 460 px wide at desktop, wide figures still fill the column, and a phone-width check shows no horizontal scroll.

Docs figures now render at their CSS-pixel size: a remark plugin halves the 2× PNG width and height, and wide figures still cap at the column. Built site: details 460 px, wide 823 px (the column), no phone scroll. Quick gate green; PR #6509 queued in the lane; visual check done. Follow-up draft #8653. Memory: none applied.