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

[George-iam]

Summary

• New axme_sdk/mesh.py with MeshClient class
• Accessible via client.mesh property (lazy init)
• Background heartbeat thread (daemon, 30s interval)
• Metric buffering - flushed with each heartbeat
• Agent list/detail with health status
• Kill/resume for instant agent isolation
• Audit event log

Usage

fromaxmeimportAxmeClient, AxmeClientConfigclient=AxmeClient(AxmeClientConfig(api_key="axme_sa_..."))
client.mesh.start_heartbeat() # background livenessclient.mesh.report_metric(success=True, latency_ms=230, cost_usd=0.02)
agents=client.mesh.list_agents()
client.mesh.kill(address_id)

Test plan

• 74 unit tests passed
• Manual test with staging gateway