test: temp CI — Windows no-git default-user flow repro (DO NOT MERGE)

Removes all other workflows on this branch and adds a single
windows-latest job that:
  - disables the runner's preinstalled Git for Windows
  - installs xlings via the official quick_install.ps1 one-liner
  - runs 'xlings install mcpp', 'mcpp new/build', 'mcpp self init --force'
  - probes 'git --version' with XLINGS_HOME redirected to ~/.mcpp/registry
    to confirm the xvm git-shim / redirected-XLINGS_HOME root cause

Author: Sunrisepeak