Part of #617.

What to build

The plugin skeleton and the command that creates a learner workspace.

plugins/l/learn/
  xmake.lua              package("learn") set_kind("plugin")
  src/
    xmake.lua            task("learn") + set_menu
    main.lua             entry point
    modules/             (added by later issues)
    workspace/xmake.lua  copied into the learner's workspace

xmake learn init [dir] creates:

<dir>/
  xmake.lua        add_repositories(...) + add_requires("enginesquared", {configs = {core_only = true}})
  exercises/       copied from the plugin payload
  .progress/       state, gitignored
  .learn-mode      "tutor"

The generated xmake.lua derives one target per exercise directory from the numeric folder prefixes, each set_default(false) so nothing broken ever builds by accident. No git anywhere in the flow.

Done when

• xmake plugin --install of this repository registers a working xmake learn
• xmake learn init dir produces a workspace that configures and builds a placeholder exercise on Linux, macOS and Windows
• the workspace needs no xrepo add-repo from the learner

Depends on

EngineSquared/xrepo#1, #618, #619