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>

generate-marketplace.py

@@ -16,6 +16,7 @@ marketplace_file = script_dir / ".claude-plugin" / "marketplace.json"
 
 # Skill directories (label, directory name)
 SKILL_DIRS = [
+    ("core",        "skills-core"),
     ("dev",         "skills-dev"),
     ("req",         "skills-req"),
     ("ops",         "skills-ops"),