docs: add CHANGELOG, RELEASE guide, and LICENSE

Author: euxx

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [1.0.0] - 2026-03-03
+
+- 🔔 Real-time unread count badge and desktop notifications
+- 📋 Issues, PRs, Releases, and all notification types with rich details
+- ✅ Mark as read per notification, per repository, or all at once
+- 🔐 OAuth Device Flow or Personal Access Token authentication
+- ✨ Clean, minimal UI — feature-rich without the clutter
+- 🎨 Light / Dark / System theme with adjustable popup width

LICENSE

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+MIT License
+
+Copyright (c) 2026 GitHub Notifier Pro contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

RELEASE.md

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+# Release Guide
+
+## Steps to Release a New Version
+
+1. Update CHANGELOG.md:
+   - Add version entry: `## [X.Y.Z] - YYYY-MM-DD` with changes
+
+2. Update version in `package.json` and `manifest.json`:
+   ```sh
+   # Edit both files to set "version": "X.Y.Z"
+   ```
+
+3. Commit changes:
+   ```sh
+   git add CHANGELOG.md package.json manifest.json manifest-firefox.json
+   git commit -m "chore: update version to vX.Y.Z"
+   git push origin main
+   ```
+
+4. Run the release workflow:
+   ```sh
+   gh workflow run release.yml
+   ```
+   This will run tests, build Chrome & Firefox packages, and create a GitHub Release with the zip files.
+
+5. Update the release notes on GitHub if needed:
+   ```sh
+   gh release edit vX.Y.Z --notes "Release notes here"
+   ```