Part of #617.

Problem

The whole delivery model rests on xmake addon --install, added in xmake 3.1.0. Before anything is built on top of it, it has to be proven on the three platforms the project supports — and the exact source layout it expects has to be pinned down.

What to do

Build a throwaway plugin that registers a xmake hello task and verify, on Linux, macOS and Windows:

• xmake addon --install /local/dir works (the development loop)
• xmake addon --install github:Org/repo works, including #branch
• the plugins/<first-letter>/<name>/ package layout is what a git-url install expects, and where src/xmake.lua and main.lua must sit
• --list, --remove and re-install behave (re-install is the update path)
• a plugin can ship and read arbitrary payload files (the curriculum) via os.scriptdir()
• xmake addon --install from a private repository, or a documented failure

Done when

Findings are written into the epic, including the minimum xmake version. Note the engine README currently declares a 3.0.x floor; the track needs 3.1.0+, and that difference must be documented for learners.