feat(guild-discovery): add purpose column to guild_nodes + cli set-purpose

Adds a free-form 'purpose' text field on GuildNode so admins can describe
what each guild is for (debate broadcasts / agent triage / dev sandbox /
etc.). Surfaced through:

  - GET /api/auth/me/guilds   (existing user-facing list)
  - GET /api/nodes             (existing admin-facing list)
  - new cli: node set-purpose --node-id X --purpose 'text'

Admin-only writes via CLI (HangmanLab pattern) — never HTTP. Empty string
clears. TypeORM synchronize auto-adds the column on first startup.

Motivates: agents discover the right guild by intent (purpose substring
match) before picking a channel, so dialectic/announce workflows don't
need hardcoded guild ids.

src/nodes/node-admin.service.ts

@@ -1,4 +1,4 @@
-import { ConflictException, Injectable } from '@nestjs/common';
+import { ConflictException, Injectable, NotFoundException } from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
 import { Repository } from 'typeorm';
 import bcrypt from 'bcryptjs';
@@ -55,5 +55,26 @@ export class NodeAdminService {
       apiKey: rawApiKey,
     };
   }
+
+  // Admin-only via cli (never HTTP): set the free-form purpose string on
+  // a guild node. Pass an empty string to clear it (null).
+  async setPurpose(nodeId: string, purpose: string) {
+    const node = await this.nodeRepo.findOne({ where: { nodeId } });
+    if (!node) throw new NotFoundException(`node ${nodeId} not found`);
+    const trimmed = String(purpose ?? '').trim();
+    node.purpose = trimmed === '' ? null : trimmed;
+    const saved = await this.nodeRepo.save(node);
+    await this.audit.write({
+      action: 'node.set_purpose',
+      targetType: 'node',
+      targetId: saved.nodeId,
+      detail: JSON.stringify({ purpose: saved.purpose, via: 'cli' }),
+    });
+    return {
+      nodeId: saved.nodeId,
+      name: saved.name,
+      purpose: saved.purpose,
+    };
+  }
 }