docs(plan): demote 20260424_lightweight_pluggable_architecture to evidence source

Add deprecation headers pointing to canonical sources:
- design.md → blueprint/design.md + docs/adr/
- background.md → blueprint/background.md
- tasks.md → blueprint/tasks.md

Files are preserved for historical context during implementation phase.
Not to be used as current machine truth reference.

Author: Li-Sanze

.sopify-skills/plan/20260424_lightweight_pluggable_architecture/background.md

@@ -1,4 +1,6 @@
-# Background: Sopify 轻量化可插拔架构
+# Background: Sopify 轻量化可插拔架构（已降级为历史证据源）
+
+> **⚠️ 本文件已于 2026-05-01 降级。** Canonical 背景见 `../../blueprint/background.md`。
 
 > **战略方向 (2026-04-26)**：Sopify 已确认 Protocol-first / Runtime-optional 方向。核心价值层定位为 `.sopify-skills/` 文件协议和 schema，runtime 降级为可选增强层与参考实现。详见 design.md §1.2 及 ADR-016。
 

.sopify-skills/plan/20260424_lightweight_pluggable_architecture/design.md

@@ -1,4 +1,9 @@
-# Design: Sopify 总纲
+# Design: Sopify 总纲（已降级为历史证据源）
+
+> **⚠️ 本文件已于 2026-05-01 降级。**
+> Canonical 设计基线已迁移至 `../../blueprint/design.md`。
+> ADR 实体已迁移至 `../../../docs/adr/ADR-013.md`、`ADR-016.md`、`ADR-017.md`。
+> 本文件保留为历史证据源，供实现阶段追溯决策上下文。不应作为当前 machine truth 引用。
 
 ## §1 战略方向
 

.sopify-skills/plan/20260424_lightweight_pluggable_architecture/tasks.md

@@ -1,4 +1,6 @@
-# Tasks: Sopify 总纲
+# Tasks: Sopify 总纲（已降级为历史证据源）
+
+> **⚠️ 本文件已于 2026-05-01 降级。** Canonical 任务优先级见 `../../blueprint/tasks.md`。
 
 ## 子任务包索引