fix: 让 LICENSE 被 GitHub licensee 识别为 CC BY 4.0

之前的 attribution preamble 导致 GitHub 把 license 检测为
NOASSERTION（fuzzy match 阈值不到 95%）。改为纯 SPDX 标准
文本，attribution 信息后续放到 README。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: walter201230

LICENSE

@@ -1,14 +1,3 @@
-Copyright (c) 2017-2026 walter201230 <https://github.com/walter201230>
-
-The "小白学 Python" tutorial (markdown chapters, code examples, and
-accompanying images in this repository) is licensed under the Creative
-Commons Attribution 4.0 International License (CC BY 4.0).
-
-To view a copy of this license, visit:
-https://creativecommons.org/licenses/by/4.0/
-
-================================================================================
-
 Creative Commons Attribution 4.0 International
 
  Creative Commons Corporation (“Creative Commons”) is not a law firm and does not provide legal services or legal advice. Distribution of Creative Commons public licenses does not create a lawyer-client or other relationship. Creative Commons makes its licenses and related information available on an “as-is” basis. Creative Commons gives no warranties regarding its licenses, any material licensed under their terms and conditions, or any related information. Creative Commons disclaims all liability for damages resulting from their use to the fullest extent possible.