feat(sdk+cli): bridge peer — forward a topic between two meshes

A bridge holds memberships in two meshes and relays messages on a
single topic between them. Federation-lite without a broker-to-broker
protocol.

SDK additions:
- Bridge class (start, stop, EventEmitter for forwarded/dropped/error)
- MeshClient.joinTopic / leaveTopic / createTopic methods
- Loop prevention: plaintext hop counter prefix __cmh<n>: with maxHops
  default 2; echo guard via senderPubkey == own session pubkey

CLI additions:
- claudemesh bridge run <config.yaml> long-lived process
- claudemesh bridge init prints config template
- Zero-dep YAML parser for the flat bridge config shape

The hop prefix is visible in message bodies — minor wart, fixed in
v0.3.0 by moving loop tracking into broker primitives.

SDK kept as devDependency since Bun bundles it into dist; no impact
on npm publish or runtime resolution.

Spec: .artifacts/specs/2026-05-02-v0.2.0-scope.md

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

packages/sdk/src/client.ts

@@ -365,6 +365,32 @@ export class MeshClient extends EventEmitter {
     this.ws.send(JSON.stringify({ type: "set_status", status }));
   }
 
+  // --- Topics (v0.2.0) ---
+  // Conversational primitive within a mesh. To receive topic-tagged
+  // messages, you must subscribe via `joinTopic`.
+
+  async createTopic(args: {
+    name: string;
+    description?: string;
+    visibility?: "public" | "private" | "dm";
+  }): Promise<void> {
+    if (!this.ws || this.ws.readyState !== WebSocket.OPEN) return;
+    this.ws.send(JSON.stringify({ type: "topic_create", ...args }));
+  }
+
+  async joinTopic(
+    topic: string,
+    role?: "lead" | "member" | "observer",
+  ): Promise<void> {
+    if (!this.ws || this.ws.readyState !== WebSocket.OPEN) return;
+    this.ws.send(JSON.stringify({ type: "topic_join", topic, role }));
+  }
+
+  async leaveTopic(topic: string): Promise<void> {
+    if (!this.ws || this.ws.readyState !== WebSocket.OPEN) return;
+    this.ws.send(JSON.stringify({ type: "topic_leave", topic }));
+  }
+
   // --- Internals ---
 
   private makeReqId(): string {