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/mkdir.fish

@@ -1,7 +1,20 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Execute mkdir
+# SYNOPSIS
+#   mkdir [args...]
+#
+# DESCRIPTION
+#   Interactive wrapper around mkdir that calls _fish_mkdir_p for each
+#   directory argument to display created path components. Falls back to
+#   command mkdir -p when flags (e.g. -m 755) are present, and to plain
+#   command mkdir in non-interactive contexts.
+#
+# ARGUMENTS
+#   args...  Directories to create, or flags passed through to command mkdir
+#
+# EXAMPLE
+#   mkdir ~/projects/myapp/src
 function mkdir --description 'Execute mkdir'
     if status is-interactive
         # Fall back to command mkdir -p when flags are present (e.g. -m 755)