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/check_fish_deps.fish

@@ -1,7 +1,15 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Thin wrapper kept for backwards compatibility.
+# SYNOPSIS
+#   check_fish_deps
+#
+# DESCRIPTION
+#   Backwards-compatibility wrapper that delegates to fish-deps status to
+#   report which fish shell dependencies are installed or missing.
+#
+# EXAMPLE
+#   check_fish_deps
 function check_fish_deps --description 'Check all fish-related dependencies'
     fish-deps status
 end