fix(a11y): skip link, labelled nav, table scope and 320px reflow on the website

[JustMaris]

From the keyboard/reflow probe on the website:

• Skip-to-content link, focusable <main id=main>, labelled main nav.
• Broker directory: scope=col on headers; the table scrolls inside a labelled, focusable region instead of the page (it overflowed 375px at 320px width); the Guides-only checkbox is a 24px target.
• Docs: images no longer overflow (the new Automation screenshot was 2560px wide on a phone), long inline code wraps, code blocks and wide tables scroll in their own box. Every page checked now has zero horizontal overflow at 320px.
• Adds scripts/a11y/probe.mjs (tab stops, focus rings, mouse-only controls, skip link, small targets, reflow). Informational, not in CI.

axe scan: no violations. Left alone: markdown-generated docs tables have th without scope (axe accepts them; Hugo has no table render hook), and the web UI's small checkboxes and some 20px text links, which are web UI scope.