Port ImageEditor: Local ShareX -> XerahS

Use this workflow whenever XerahS needs to catch up with the current ShareX.ImageEditor state from the local ShareX repo.

Source of truth

On the known workstations, do not clone ShareX again. The local ShareX checkout is the upstream reference.

ShareX and XerahS are not always siblings. Resolve $ShareXRepo and $XerahSRoot independently. The editor cwd is often C:\WINDOWS\system32 or another unrelated folder — never run git there.

Session paths (resolve first)

User-specified paths win. Then probe, in order, until git rev-parse --is-inside-work-tree succeeds.