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/_fish_deps_detect_pm.fish

@@ -1,7 +1,16 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Detect the first available system package manager.
+# SYNOPSIS
+#   _fish_deps_detect_pm
+#
+# DESCRIPTION
+#   Detects and prints the name of the first available system package manager
+#   from the priority list: paru, yay, pacman, apt, brew, pkg, dnf, yum.
+#   Prints an empty string if none are found.
+#
+# EXAMPLE
+#   set pm (_fish_deps_detect_pm)
 function _fish_deps_detect_pm
     for pm in paru yay pacman apt brew pkg dnf yum
         if type -q $pm