feat(tools): fabric-guild-list returns serviceEndpoint for announce targets

FabricSession.guilds type + fabric-guild-list response now carry the
new serviceEndpoint field (added to GuildNode in Fabric.Backend.Center).
The tool description teaches agents the distinction: 'endpoint' is the
client-facing URL (which they themselves use to call the guild from
plugin context), 'serviceEndpoint' is what to plumb into
dialectic_propose_topic's announce_guild_base_url so the dialectic
backend can dial it from inside the deployment.

Fixes bug #2 from the first e2e debate run: agent-supplied
'http://server.t3:7002' wasn't backend-reachable; agent now
supplies 'http://fabric-backend-guild:7002' via serviceEndpoint
and broadcasts actually land.

src/fabric-client.ts

@@ -10,6 +10,13 @@ export type FabricSession = {
     nodeId: string;
     name: string;
     endpoint: string;
+    // Service-to-service endpoint (e.g. http://backend-guild:7002) — what
+    // an in-deployment backend uses to reach this guild. Distinct from
+    // `endpoint` which is the client/browser-facing URL. Plumb this into
+    // `announce_guild_base_url` on dialectic_propose_topic so the
+    // dialectic-backend can actually dial it. Null when the admin hasn't
+    // configured it.
+    serviceEndpoint?: string | null;
     status: string;
     // free-form description of this guild's role; admin-set on Center.
     // null when the admin hasn't filled it in yet.