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/__insert_previous_path_head.fish

@@ -1,7 +1,16 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Inserts the directory path (dirname) of the last argument from the previous command
+# SYNOPSIS
+#   __insert_previous_path_head
+#
+# DESCRIPTION
+#   Inserts the directory component (dirname) of the last argument from the
+#   previous history command into the current commandline. Used as a keybinding
+#   helper to quickly reuse a directory path.
+#
+# EXAMPLE
+#   # Bind to a key to insert the directory from the last command.
 function __insert_previous_path_head
     # Get the last command tokens
     set -l tokens (string split -n " " -- $history[1])