feat(doctor): add legend line and --json output (#348)

The ok/warn/off symbols had no explanation for non-technical users;
--json gives agents and scripts a machine-readable health check.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent_reach/doctor.py

@@ -34,6 +34,7 @@ def format_report(results: Dict[str, dict]) -> str:
     lines = []
     lines.append("[bold cyan]Agent Reach 状态[/bold cyan]")
     lines.append("[cyan]" + "=" * 40 + "[/cyan]")
+    lines.append("图例：[green]✅[/green] 可用  [yellow][!][/yellow] 已装但需配置/登录  [red][X][/red] 未安装")
 
     ok_count = sum(1 for r in results.values() if r["status"] == "ok")
     total = len(results)