docs(functions): add # CATEGORY to documented function headers

Records each documented function's manual category in its own header, so
Section 5 can later be generated from source instead of hand-maintained
alongside it. Values reproduce the current grouping in
docs/manual/05-functions/ exactly; no documentation changes meaning here.

Four functions are skipped because they have no header at all yet
(branch, fc, gitup, sudo-toggle); they get one in the merge that follows.

functions/lock.fish

@@ -1,6 +1,9 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
+# CATEGORY
+#   07-system-and-monitoring
+#
 # SYNOPSIS
 #   lock
 #