feat(broker+api): every mesh ships with a default #general topic

The web chat surface needed a guaranteed landing room — a topic that
exists for every mesh from creation onward so the dashboard always has
somewhere to drop the user. #general is the convention; ephemeral DMs
remain ephemeral (mesh.message_queue) so agentic privacy is unchanged.

Three hooks plus a backfill:

- packages/api/src/modules/mesh/mutations.ts — createMyMesh now calls
  ensureGeneralTopic() right after the mesh insert. New helper is
  idempotent via the unique (mesh_id, name) index.
- apps/broker/src/index.ts — handleMeshCreate (CLI claudemesh new)
  inserts #general + subscribes the owner member as 'lead' in the
  same handler.
- apps/broker/src/crypto.ts — invite-claim flow auto-subscribes the
  newly minted member to #general as 'member', defensively ensuring
  the topic exists if predates this change.
- packages/db/migrations/0024_general_topic_backfill.sql — one-shot
  backfill: creates #general for every active mesh that doesn't have
  one, subscribes every active member, and marks the mesh owner as
  'lead' based on owner_user_id == member.user_id. Idempotent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/broker/src/index.ts

@@ -5945,6 +5945,22 @@ async function handleCliMeshCreate(req: IncomingMessage, res: ServerResponse, st
       VALUES (${memberId}, ${meshId}, ${auth.userId}, ${body.pubkey}, ${body.name + "-owner"}, ${"admin"})
     `);
 
+    // Auto-create the conventional #general topic + subscribe the owner.
+    // Idempotent via unique (mesh_id, name) — re-running is a no-op.
+    const generalTopicId = generateId();
+    await db.execute(sql`
+      INSERT INTO mesh.topic (id, mesh_id, name, description, visibility, created_by_member_id)
+      VALUES (${generalTopicId}, ${meshId}, ${"general"}, ${"Default mesh-wide channel. Every member can read and post."}, ${"public"}, ${memberId})
+      ON CONFLICT (mesh_id, name) DO NOTHING
+    `);
+    await db.execute(sql`
+      INSERT INTO mesh.topic_member (topic_id, member_id, role)
+      SELECT t.id, ${memberId}, ${"lead"}
+      FROM mesh.topic t
+      WHERE t.mesh_id = ${meshId} AND t.name = ${"general"}
+      ON CONFLICT (topic_id, member_id) DO NOTHING
+    `);
+
     writeJson(res, 200, { id: meshId, slug, name: body.name, member_id: memberId });
     log.info("mesh-create", { route: "POST /cli/mesh/create", slug, user_id: auth.userId, latency_ms: Date.now() - started });
   } catch (e) {