feat: Agent Mesh module (heartbeat, health, kill switch)

[George-iam]

Summary

• New src/mesh.ts with MeshClient class
• Accessible via client.mesh property (lazy init)
• Background heartbeat via setInterval (unref'd, won't block exit)
• Metric buffering - flushed with each heartbeat
• Agent list/detail, kill/resume, events

Usage

import { AxmeClient } from "@axme/axme";

const client = new AxmeClient({ apiKey: "axme_sa_..." });
client.mesh.startHeartbeat();
client.mesh.reportMetric({ success: true, latencyMs: 230, costUsd: 0.02 });
const agents = await client.mesh.listAgents();
await client.mesh.kill("addr_...");

Test plan

• 86 unit tests passed
• Build clean (tsc)