refactor: 移除顶层 defaults 配置段，简化为 target 显式字段 > 代码内置默认值

- 移除 DefaultsConfig 类型、ProbeConfig.defaults 字段
- 移除 CheckerSchemas.defaults、ResolveContext.defaults、CheckerValidationInput.defaults
- 更新所有 checker schema/resolve/validate 删除 defaults 合并逻辑
- 更新 config-loader 不再读取传递 defaults
- 更新测试、README、DEVELOPMENT、probes.example.yaml
- 重新生成 probe-config.schema.json（不含 defaults）
- 同步 delta specs 到主规范
- 归档 openspec change

tests/server/checker/config-loader.test.ts

@@ -190,16 +190,6 @@ describe("loadConfig", () => {
 probes:
   execution:
     maxConcurrentChecks: 5
-defaults:
-  interval: "15s"
-  timeout: "5s"
-  http:
-    headers:
-      Authorization: "Bearer token"
-    maxBodyBytes: "50MB"
-  cmd:
-    cwd: "/tmp"
-    maxOutputBytes: "10MB"
 targets:
   - name: "http-target"
     id: "http-target"
@@ -236,19 +226,18 @@ targets:
     expect(http.type).toBe("http");
     expect(http.http.url).toBe("http://example.com");
     expect(http.http.method).toBe("POST");
-    expect(http.http.headers).toEqual({ Authorization: "Bearer token" });
     expect(http.http.ignoreSSL).toBe(true);
-    expect(http.http.maxBodyBytes).toBe(52428800);
+    expect(http.http.maxBodyBytes).toBe(104857600);
     expect(http.http.maxRedirects).toBe(5);
     expect(http.expect?.status).toEqual(["2xx", 301]);
     expect(http.intervalMs).toBe(60000);
-    expect(http.timeoutMs).toBe(5000);
+    expect(http.timeoutMs).toBe(10000);
 
     const cmd = config.targets[1]! as ResolvedCommandTarget;
     expect(cmd.type).toBe("cmd");
     expect(cmd.cmd.exec).toBe("ls");
     expect(cmd.cmd.args).toEqual(["/tmp"]);
-    expect(cmd.cmd.maxOutputBytes).toBe(10485760);
+    expect(cmd.cmd.maxOutputBytes).toBe(104857600);
   });
 
   test("name 缺省时保留为 null", async () => {
@@ -526,16 +515,11 @@ targets:
     expect(config.dataDir).toBe(dataDir);
   });
 
-  test("per-target 覆盖 defaults", async () => {
+  test("per-target interval 和 timeout 覆盖全局默认", async () => {
     const configPath = join(tempDir, "override.yaml");
     await writeFile(
       configPath,
-      `defaults:
-  interval: "30s"
-  timeout: "10s"
-  http:
-    maxBodyBytes: "10MB"
-targets:
+      `targets:
   - name: "override-all"
     id: "override-all"
     type: http
@@ -799,15 +783,13 @@ targets:
     const configPath = join(tempDir, "bad-size.yaml");
     await writeFile(
       configPath,
-      `defaults:
-  http:
-    maxBodyBytes: "100TB"
-targets:
+      `targets:
   - name: "t"
     id: "t"
     type: http
     http:
       url: "http://a.com"
+      maxBodyBytes: "100TB"
 `,
     );
     await expectConfigLoadError(configPath, "无效的 size 格式");
@@ -1388,9 +1370,9 @@ targets:
     );
   });
 
-  test("defaults.http.method 触发未知字段错误", async () => {
+  test("defaults 顶层字段触发未知字段错误", async () => {
     await expectConfigError(
-      "unknown-default-method.yaml",
+      "unknown-defaults.yaml",
       `defaults:
   http:
     method: POST
@@ -1401,7 +1383,7 @@ targets:
     http:
       url: "http://example.com"
 `,
-      "defaults.http.method 是未知字段",
+      "defaults 是未知字段",
     );
   });
 
@@ -1428,11 +1410,7 @@ targets:
     const configPath = join(tempDir, "dynamic-maps.yaml");
     await writeFile(
       configPath,
-      `defaults:
-  http:
-    headers:
-      X-Default-Header: "default"
-targets:
+      `targets:
   - name: "http-test"
     id: "http-test"
     type: http
@@ -1458,7 +1436,6 @@ targets:
     const cmdTarget = config.targets[1] as ResolvedCommandTarget;
     expect(http.type).toBe("http");
     expect(cmdTarget.type).toBe("cmd");
-    expect(http.http.headers["X-Default-Header"]).toBe("default");
     expect(http.http.headers["X-Custom-Header"]).toBe("custom");
     expect(cmdTarget.cmd.env["CUSTOM_ENV_NAME"]).toBe("custom");
   });
@@ -1924,15 +1901,11 @@ targets:
     );
   });
 
-  test("tcp defaults 覆盖 banner 参数", async () => {
+  test("tcp per-target banner 参数覆盖", async () => {
     const configPath = join(tempDir, "tcp-defaults.yaml");
     await writeFile(
       configPath,
-      `defaults:
-  tcp:
-    bannerReadTimeout: 1000
-    maxBannerBytes: "8KB"
-targets:
+      `targets:
   - id: "t1"
     type: tcp
     tcp:
@@ -1949,12 +1922,12 @@ targets:
 
     const config = await loadConfig(configPath);
     const t1 = config.targets[0]! as ResolvedTcpTarget;
-    expect(t1.tcp.bannerReadTimeout).toBe(1000);
-    expect(t1.tcp.maxBannerBytes).toBe(8192);
+    expect(t1.tcp.bannerReadTimeout).toBe(2000);
+    expect(t1.tcp.maxBannerBytes).toBe(4096);
 
     const t2 = config.targets[1]! as ResolvedTcpTarget;
     expect(t2.tcp.bannerReadTimeout).toBe(3000);
-    expect(t2.tcp.maxBannerBytes).toBe(8192);
+    expect(t2.tcp.maxBannerBytes).toBe(4096);
   });
 
   test("tcp expect 未知字段抛出错误", async () => {