fix: harden system-install review findings

Require root-owned, non-group/other-writable pmg for --system install;
allow remove without that validation. Doctor checks npm resolution for
PATH precedence, uses ImpliesInterception instead of message matching,
and documents version-manager shadowing. Pass profile bin dir from the
shim manager and note that system config ignores per-user files.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/shim/file_owner_windows.go

@@ -0,0 +1,9 @@
+//go:build windows
+
+package shim
+
+import "os"
+
+func fileOwnerUID(info os.FileInfo) (uint32, bool) {
+	return 0, false
+}