add ref and AGENTS.md

AGENTS.md

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+Analyze a file format of a song.
+
+## Spec
+
+File: ./Test.pro (file ext are always .pro)
+
+- every song contains parts (name group here) (here: Verse 1, Verse 2, Chorus, ...) but could be any name
+- every group contains 1-x slides
+- every song contains different arrangements (here normal and test2) that defines the existence and the order of the groups
+- every slide CAN have another textbox which contains a translated version of the first textbox
+
+## ToDo
+
+1. analyse the file structure and find all of the described specs.
+2. test and verify if the definition is correct - there is a `all-songs` directory with lot of examples.
+3. describe the structure for future AI prompts to use these files in `spec/pp_song_spec.md` and describe the usage in the `AGENTS.md` (replace obsolet commands)
+4. write a PHP module (is later used in laravel) in `./php` which can parse a song and let get/set every aspect of structure. Use Objects here (Song, Group, Slide, Arrangement, etc)
+5. create a simple PHP cli tool, which receive a param with a song file and show the structure of the song.