feat: cac env check --json for machine-readable output

[nmhjklnm]

Summary

Add --json flag to cac env check for machine-readable output, useful for scripting and monitoring.

Proposed API

cac env check --json

Output format

{
  "env": "work",
  "claude_version": "2.1.83",
  "wrapper_active": true,
  "proxy": {
    "url": "socks5://***@host:port",
    "reachable": true,
    "exit_ip": "1.2.3.4"
  },
  "identity": {
    "uuid": "...",
    "hostname": "host-abc12345",
    "stable_id": "..."
  },
  "dns_guard": true,
  "fingerprint_hook": true,
  "telemetry_blocked": true,
  "relay": {
    "active": true,
    "port": 17890
  }
}

Use cases

• CI/CD environment validation
• Monitoring dashboards
• Scripted health checks

[nmhjklnm]

Closing as no longer planned.