Update project license to CC BY-NC-SA 4.0

[jdebacker]

Summary

This PR updates the project licensing from BSD 3-Clause to Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0), and refreshes the copyright year to 2026.

What changed

• updated the top-level LICENSE file
• changed the copyright notice to 2026
• replaced the old BSD 3-Clause text with a CC BY-NC-SA 4.0 notice
• expanded the LICENSE file with a plain-English summary of:
  • what is permitted
  • what attribution and share-alike obligations apply
  • what is not permitted, especially commercial use
  • the note that the official Creative Commons license text controls
• updated book/content/repomgt/license.md so the book’s licensing discussion reflects the project’s current license choice

Notes

• The LICENSE file now includes a readable summary plus the official Creative Commons license URL.
• The binding legal terms remain those of the official CC BY-NC-SA 4.0 license:
  https://creativecommons.org/licenses/by-nc-sa/4.0/

[jdebacker]

Addresses Issue #21