Live status badge and a static social preview card for Satsuma's status page

The last part of the plan for Satsuma's public status service was committed in a Claude Code session. The service reports whether Satsuma's chain is operational, degraded or down. This work adds a live status badge at /badge.svg that shows one of those three states, and /badge.json, a shields.io-compatible endpoint for pages that draw their own badges.

It also adds a social preview image, the picture that appears under a link to the service when it is shared on X or Telegram. The README and AGENTS.md were updated in the same change; both still described the old incidents.json workflow.

The preview image is kept static by decision. "The card is static on purpose: those networks cache preview images for days, so a live one would show stale status." The badge and the /api/status endpoint are the live parts.

A separate task remains open: showing a live status dot on satsuma.one, the explorer and this engineering site.