footerLinksRegexes badges: enroll in the Footer keyboard-selection ring (parity with artifact links)

Feature request

Footer link badges produced by the footerLinksRegexes setting render as OSC-8 hyperlinks in the footer row, but are mouse-only. The footer already has a keyboard selection ring — Down activates it, selected items render reverse-video, Enter opens, Backspace dismisses artifact links — and it enrolls task indicators and Artifact-tool links, but skips regex badges entirely.

Verified empirically on v2.1.218 (tmux capture with a live background shell + a regex badge in the same row): Down selects the 1 shell indicator; no key (Left/Right/Tab/Down) ever moves selection onto the regex badge, which stays a plain underlined hyperlink.

Ask: enroll footerLinksRegexes badges in the existing Footer keybinding context, so footer:next/previous reach them and footer:openSelected opens them — exactly like artifact links today.

Why it matters: in terminals without OSC-8 support (stock macOS Terminal.app), these badges are currently unopenable by any means; even with OSC-8 support, keyboard-driven users select artifact links daily via Down+Enter and expect the same for configured badges. The selection/open machinery already exists in that row — this is enrollment, not new UI.