docs(license): replace the license with the Zavage Software Freeware License 1.0

The old LICENSE.txt was a stock business EULA. It allowed use only "for your
internal business purposes", banned copying and reverse engineering of code we
publish, named "Zavage Software inc." as licensor, and claimed users agreed by
installing.

The new text lets anyone use the extension free of charge, for any purpose,
and withholds republishing, forks and reuse of the code. It also covers
versions 1.0 to 1.3, which AMO shows under one shared license record. The
README, CHANGELOG and package.json follow suit.

The text went through three multi-agent review passes. The decisions behind
it and the AMO publishing steps are in
docs/superpowers/specs/2026-10-19-license-design.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -7,7 +7,7 @@ It uses no API key, custom server, or intermediary handler.
 **ChatGPT may require you to sign in. If your prompt is not sent automatically,
 press Enter or click Send in ChatGPT.**
 
-This is an unofficial extension provided by Zavage Software Inc.
+This is an unofficial extension by Zavage Software.
 
 - [Firefox Add-ons listing](https://addons.mozilla.org/en-US/firefox/addon/search-with-chatgpt/)
 - [Project website](https://zavage-software.com/portfolio/search-with-ChatGPT)
@@ -15,7 +15,9 @@ This is an unofficial extension provided by Zavage Software Inc.
 - [Feedback or report a bug](https://zavage-software.com/contact-us)
 - [Zavage Software](https://zavage-software.com)
 
-See `LICENSE.txt` for licensing terms.
+Anyone may use the extension free of charge, for any purpose. Republishing it,
+sharing changed versions or reusing its code needs permission. You may read the
+source, but it isn't open source. See `LICENSE.txt` for the full terms.
 
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