v6.0.1 -- Bootstrap Hardening

Fixes four defects that shared one signature: a fallback path that could
never run, hidden behind a reassuring message.

- Successful git clones were reported as failures. `2>&1 | Out-Null` under
  $ErrorActionPreference = "Stop" turned git's progress output into a
  terminating error, so every Windows install fell through to the ZIP mirror
  and therefore to Expand-Archive -- the original bug report. On Linux the
  same class of bug wore the opposite face: `set -e` plus a bare `git clone`
  aborted comfyui.sh outright, making all three fallback sources dead code.

- `::` comments inside a cmd `( )` block are executed, not ignored. Seventeen
  of them printed eight "cannot find the drive specified" lines on every
  install and swallowed a real command. The uv logic moved to a subroutine.

- Install.bat printed "[INFO] uv installed." whether or not uv.exe existed.
  Extraction now cascades System32\tar.exe -> .NET ZipFile -> Expand-Archive
  and every tier is verified against the file on disk.

- SageAttention 2.2.0 had not installed on Windows since v5.1.17: resolve()
  never tried the platform-prefixed abi3 keys the JSON actually ships, so
  users silently got sageattention 1.0.6 from PyPI while running launchers
  that pass --use-sage-attention.

Also removes Migrate-from-PS.ps1 and the sync-oneliner job: a duplicate kept
aligned by CI that could never push, and had silently drifted in both
directions.