feat!: probes connect to the backend directly

A probe now links to the backend like an agent (probe.json carries the
server address). The backend sends the agent list as desired state and the
probe reports its platform and build in its own hello. The agent no longer
relays the list or the probe platform. Existing probes must be re-created.

backend/probe/src/Monitor.Probe/ProbeConfig.cs

@@ -15,15 +15,15 @@ public sealed class ProbeFile
     [JsonPropertyName("private_key")]
     public string PrivateKey { get; set; } = "";
 
-    /// <summary>首次连接用的 Agent；连上之后以 <c>agents.json</c>（最新的列表）为准。</summary>
-    [JsonPropertyName("bootstrap")]
-    public List<BootstrapAgent> Bootstrap { get; set; } = [];
+    /// <summary>面板的地址（http 或 https）。Probe 直接连它：身份、Agent 列表、更新都从这里来。</summary>
+    [JsonPropertyName("server")]
+    public string Server { get; set; } = "";
 
     [JsonPropertyName("limits")]
     public ProbeLimits Limits { get; set; } = new();
 }
 
-public sealed class BootstrapAgent
+public sealed class KnownAgent
 {
     [JsonPropertyName("agent_id")]
     public string AgentId { get; set; } = "";
@@ -54,12 +54,12 @@ public sealed class ProbeLimits
 [JsonSerializable(typeof(AgentsCache))]
 public sealed partial class ProbeConfigContext : JsonSerializerContext;
 
-/// <summary><c>agents.json</c>：最近一次从 Agent 同步到的列表（配置，不是测量数据）。</summary>
+/// <summary><c>agents.json</c>：最近一次从面板同步到的 Agent 列表（配置，不是测量数据）；面板连不上时 Probe 靠它继续测量。</summary>
 public sealed class AgentsCache
 {
     [JsonPropertyName("version")]
     public long Version { get; set; }
 
     [JsonPropertyName("agents")]
-    public List<BootstrapAgent> Agents { get; set; } = [];
+    public List<KnownAgent> Agents { get; set; } = [];
 }