feat(core): add configuration declaration registry

配置读取框架的内核侧抽象：插件用带 config/env/default/autoEnable/secret
tag 的结构体声明自己读哪些字段，注册表按 key 归集并对重复声明做一致性
校验，为后续按声明解析与门禁求值提供基础。

backend/core/extpoints/config_resolve.go

@@ -0,0 +1,25 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package extpoints
+
+import "sort"
+
+// Entries returns the effective configuration as redacted, key-sorted entries.
+func (r *ConfigRegistry) Entries() []ConfigEntry {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	keys := append([]string(nil), r.order...)
+	sort.Strings(keys)
+
+	out := make([]ConfigEntry, 0, len(keys))
+	for _, key := range keys {
+		d := r.decls[key]
+		out = append(out, ConfigEntry{
+			Key: d.key, PluginID: d.pluginID, Env: d.env,
+			Origin: r.origins[key], Value: "pending",
+		})
+	}
+	return out
+}