mDNS / zero-conf discovery for workers on the local network

[jaylfc]

Today joining a worker to the cluster requires the user to copy the controller URL into the worker's install prompt. On a home LAN this is clunky — the controller should advertise itself via mDNS so fresh workers can auto-discover and offer to attach.

What needs to ship:

• Controller publishes `_tinyagentos._tcp.local` service with controller URL, fingerprint, and hardware tier
• Worker install script scans for existing controllers on the LAN during interactive install and offers a list to pick from
• Worker daemon optionally auto-attaches to a specific controller fingerprint if configured
• Graceful fallback to manual URL entry when mDNS is unavailable (containerised installs, restricted networks, CGNAT)
• No mDNS outside the LAN — remote attach stays via pairing code + mesh

User impact today: adding a worker to a home cluster is an unnecessary manual step with a failure mode (typo'd URL) that looks confusing to beginners.