fix(skills): open reparse points in Windows file ID query and stabilize tunneled test

Add FILE_FLAG_OPEN_REPARSE_POINT to the Windows CreateFile call in
skillPathFileIdentityImpl so symlinks are opened as reparse points
rather than followed to their target. Staged symlinks carry relative
targets computed for the final destination, which may not resolve from
the staging directory; following them caused CreateFile to fail,
yielding an empty file ID that rejected publication and broke canonical
skill layout migration on Windows.

Make skillPathSameFileIdentity a seam variable so the tunneled
replacement test can deterministically simulate the identity change on
Unix. On tmpfs (used by Linux CI runners), os.SameFile can return true
for a recreated file due to inode reuse, making the test flaky. On
Windows the swap is a no-op because skillPathIdentityProven compares
file IDs from GetFileInformationByHandle and ignores
skillPathSameFileIdentity.

internal/upgrade/skill_publication_identity_darwin.go

@@ -17,7 +17,7 @@ func skillPathFileIncarnationImpl(info os.FileInfo) string {
 		stat.Ctimespec.Sec, stat.Ctimespec.Nsec, stat.Birthtimespec.Sec, stat.Birthtimespec.Nsec)
 }
 
-func skillPathSameFileIdentity(left, right os.FileInfo) bool {
+func skillPathSameFileIdentityImpl(left, right os.FileInfo) bool {
 	return os.SameFile(left, right)
 }