Keep Thor routing and availability state live

[wschenk]

Closes #467

Summary

• terminate and reconnect supplier sockets after two missed Exchange heartbeats
• exclude disconnected machine Offers from /v1/models and mark returned models available
• show explicit availability in the public Exchange component
• add a Clerk-admin-only supplier connections component with connection age, in-flight work, and last disconnect reason
• update agent-facing model discovery documentation and production serving checks

Production incident

Thor retained a half-open WebSocket for roughly 54 hours. Its local runtimes were healthy but Mycel could not route to it. The dialer has been safely restarted and fresh ping/pong traffic is active. This change prevents recurrence and makes status visible.

Verification

• pnpm test:run — 256 files, 3,079 tests passed
• pnpm typecheck
• focused ESLint and JavaScript syntax checks
• account browser integration — playground, admin grants, and connected Thor component passed
• public shell browser integration — availability indicator passed
• pnpm --dir apps/mycel-client build
• pnpm --filter @umwelten/mycel build