docs(functions): standardize all function doc blocks to UNIX man-page style

Replace all ad-hoc inline comments between license headers and function
declarations with consistent SYNOPSIS / DESCRIPTION / ARGUMENTS / RETURNS /
EXAMPLE blocks across all 99 project-owned functions/ files. No executable
logic, variable names, or exit codes were modified.

Completes Task #6 from AGENTS.md (Retroactive Function Documentation
Standardization).

functions/upgrade.fish

@@ -1,6 +1,19 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
+# SYNOPSIS
+#   upgrade
+#
+# DESCRIPTION
+#   Runs a full system upgrade via paru or yay with --noconfirm. Falls
+#   back to yay if paru is not installed.
+#
+# RETURNS
+#   0  Upgrade completed successfully
+#   1  No AUR helper (paru or yay) found
+#
+# EXAMPLE
+#   upgrade
 function upgrade --description 'Full system upgrade via paru or yay'
     set -l aur ""
     if type -q paru