dsh-notion-mcp: Connect dsh and Notion via the official Notion MCP

`dsh-notion-mcp` is a DeepSeek Harness (DSH) plugin designed to connect DSH with Notion via the official Notion MCP server. It employs the OAuth 2.0 authorization code with PKCE mechanism to enable one-time browser authorization, supports runtime dynamic client registration, and does not require pre-configured client IDs or secrets. The plugin handles token acquisition, automatic refresh, and atomic credential storage to ensure security. Upon successful authorization, Notion tools such as search, pages, databases, and comments are mounted to the DSH agent in the form of `mcp__notion__*`. The plugin requires Node.js version ^22.19.0 or >=24.0.0 (Node 23 is not supported) and has been verified.