refactor: ai-proj-plugin 移至 skills-core，合并冗余 session 插件

- ai-proj-plugin 作为 MCP 基础设施移入 skills-core/
- 删除 4 个冗余 session 子插件（save/reload/read/search），
  session-plugin 已包含全部功能
- 48 → 44 插件

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

skills-core/ai-proj-plugin/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "ai-proj-plugin",
+  "description": "AI project management via REST API. Works out of the box!",
+  "version": "2.0.1",
+  "author": {
+    "name": "qiudl"
+  }
+}