Add GPL-2.0+ LICENSE file

[Copilot]

Summary

Adds an official GPL-2.0+ LICENSE file to the repository to clarify the project's licensing and ensure compliance with GPL text distribution requirements.

Background

As noted in #228, the license was previously clarified as GPL, and composer.json already declares the license as GPL-2.0+. However, the repository was missing an actual LICENSE file containing the full license text.

Changes

• Added LICENSE file with the complete GNU General Public License Version 2 text
• The GPL-2.0+ license designation means GPL version 2 or any later version, giving users flexibility while maintaining WordPress compatibility

Benefits

• ✅ Makes the license immediately clear to anyone discovering this repository
• ✅ Ensures anyone who forks and redistributes the project remains in compliance with GPL text distribution requirements
• ✅ Aligns with WordPress ecosystem standards (WordPress itself is GPL-2.0+)
• ✅ Matches the existing license declaration in composer.json

Fixes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a license file</issue_title>
<issue_description>Hi, thanks for this project!

It looks like the license was clarified as GPL in #228. Do you mind adding a license file with the text of the GPL? This will make it clearer for someone that finds this repo and will ensure anyone who forks and redistributes it remains in compliance with the GPL-text distribution requirements of the license.

I would have created a PR for this but I'm not actually sure if it should be GPLv2+ or GPLv3+.</issue_description>

<agent_instructions>Add a license file for WP compatible GPL v2+</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@joehoyle Thanks for pointing this out @simonft

Fixes #314

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