Add NoteGen by @codexu

[codexu]

Add a project

1. Project URL: https://github.com/codexu/note-gen
2. Updated contents.json instead of README
3. One project in this pull request
4. Screenshot included
5. Description does not repeat “iOS” or “open source”
6. Commit title follows Add app-name by @github-username
7. Entry follows the approved format

NoteGen is a cross-platform Markdown notes app with an iOS TestFlight build. This entry places it in the Notes category and links its GPL-3.0 source, homepage, and existing product screenshot.

Checks

• jq empty contents.json
• ruby .github/osia_validate_categories.rb
• Screenshot and homepage return HTTP 200
• git diff --check

[dkhamsing]

Thank you