feat(mdns): announce this site's local presence for agent local-discovery

The announcer half of AGENT_LOCAL_DISCOVERY_SPEC.md / MULTI_SITE_SPEC.md
Appendix B -- advertises which public hostnames this site fronts (and at
what local IP) via mDNS, so a theta-agent on the same LAN segment with
prefer_local_directory enabled can skip the relay/WAN path.

Opt-in via THETA_LOCAL_DISCOVERY_HOSTS (comma-separated); no-op if unset,
so this changes nothing for an install that doesn't configure it. Only
ever advertises which hostnames map to which local IP -- no identity/
trust information -- consistent with the hard rule on the listening side
(theta-agent) that local-discovery may change DNS resolution but must
never touch certificate validation.

Verified end-to-end against the real theta-agent Go binary: announce,
discover, apply, and clean revert on disappearance all confirmed working
over real multicast between two containers.

nodejs/package.json

@@ -29,6 +29,7 @@
     "@simpleworkjs/oidc-client": "^1.0.0",
     "@simpleworkjs/orm": "^0.2.8",
     "bcrypt": "^6.0.0",
+    "bonjour-service": "^1.4.4",
     "bootstrap": "^5.3.8",
     "compression": "^1.8.1",
     "ejs": "^3.1.10",