Add csharp-library-rules.md: unified System.Web namespace documentation

[Copilot]

csharp-library-rules.md was absent; the System.Web directory lacked any documented usage rules, and style was inconsistent.

Changes

• New file: csharp-library-rules.md — covers 11 System.Web types with a uniform per-entry structure:
  • Namespace — fully qualified location
  • Description — one-line summary
  • Rules — prescriptive bullet guidelines (security, threading, lifecycle)
  • Example — fenced C# code block

Types covered

HttpContext · HttpRequest · HttpResponse · HttpServerUtility · HttpApplication · IHttpHandler · IHttpModule · HttpCookie · HttpSessionState · HttpRuntime · HttpUtility

Example entry structure

// HttpCookie — always set security flags
HttpCookie cookie = new HttpCookie("sessionId", token)
{
    HttpOnly = true,
    Secure   = true,
    Expires  = DateTime.UtcNow.AddHours(1)
};
HttpContext.Current.Response.Cookies.Add(cookie);

A General Rules section at the end covers cross-cutting concerns: System.Web is .NET Framework only (not .NET 5+), all HttpRequest input must be validated, and all HTML output must be encoded.

Original prompt

csharp-library-rules.md System.Web目录文件清空重写风格要统一

创建自 VS Code。

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