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_unix.go

@@ -0,0 +1,16 @@
+//go:build unix
+
+package shim
+
+import (
+	"os"
+	"syscall"
+)
+
+func fileOwnerUID(info os.FileInfo) (uint32, bool) {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return 0, false
+	}
+	return uint32(stat.Uid), true
+}