refactor: centralize ABI metadata into internal/meta/meta.go

- internal/meta/meta.go: add ABIVersion, ABIVersionMin, 45 dispatch
  method IDs, SDKCompatibleVersion (single source of truth)
- internal/plugin/cabi/types.go: re-export from meta.go
- internal/plugin/cabi/loader.go: reject plugin when version > ABIVersion;
  pass ABIVersion in Start instead of hardcoded 1
- loader.{go,c}: comments reference meta.go as canonical

internal/plugin/cabi/loader.c

@@ -4,6 +4,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+// HOMEAGENT_ABI_VERSION 与 internal/meta/meta.go ABIVersion 同步。
+// C ABI 通过 version/version_min 协商，旧插件不受影响。
 #define HOMEAGENT_ABI_VERSION 1
 
 // PluginAPI — provided by the plugin