refactor(guild-discovery): drop serviceEndpoint (no longer needed)

Pairs with Dialectic.Backend@5cf4302 which removes the backend-driven
broadcast that was the only consumer of serviceEndpoint. With agent-
driven recruitment broadcasts via fabric-send-message, the
service-to-service URL distinction goes away (agents use the regular
guild endpoint).

Removed:
  - GuildNode.serviceEndpoint column (TypeORM will drop on next sync)
  - GET /api/auth/me/guilds + /api/nodes response fields
  - NodeAdminService.setServiceEndpoint()
  - cli 'node set-service-endpoint' subcommand

Kept:
  - GuildNode.purpose (used by fabric-guild-list for intent-based
    channel discovery — still wanted)

src/entities/guild-node.entity.ts

@@ -20,27 +20,6 @@ export class GuildNode {
   @Column()
   endpoint!: string;
 
-  // Service-to-service endpoint: how *another service inside the same
-  // deployment* reaches this guild. Distinct from `endpoint` because
-  // that one is the client-facing URL (browser, remote openclaw plugin,
-  // anything outside compose), which lives on a different network than
-  // services co-located with the guild backend.
-  //
-  // Concrete examples:
-  //   - sim sim-guild-1: endpoint=http://server.t3:7002 (dind /etc/hosts alias),
-  //     serviceEndpoint=http://fabric-backend-guild:7002 (compose service name)
-  //   - prod t3-node: endpoint=https://fabric-api.hangman-lab.top (public CF),
-  //     serviceEndpoint=http://backend-guild:7002 (compose service name)
-  //
-  // Used by dialectic-backend (and any other in-deployment service) to
-  // post broadcasts to a guild's announce channel. Plugin's
-  // fabric-guild-list returns this so agents can plumb the right URL
-  // into announce_guild_base_url on dialectic_propose_topic.
-  // Null when the admin hasn't filled it in (broadcasts will fail until
-  // it's set; cli: `node set-service-endpoint --node-id X --endpoint URL`).
-  @Column({ type: 'varchar', length: 255, nullable: true })
-  serviceEndpoint!: string | null;
-
   // Free-form description of this guild's purpose — what it's used for,
   // who its agents/users serve. Surfaced to agents via /auth/me/guilds so
   // the cross-guild/cross-channel announce-target picker can find the