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_update.fish

@@ -1,8 +1,16 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Update all installed deps using their known install method.
-# Priority: cargo > system PM > special (fzf-update, fisher, pipx).
+# SYNOPSIS
+#   _fish_deps_update
+#
+# DESCRIPTION
+#   Updates all currently installed fish shell dependencies using their
+#   preferred method. Priority order: cargo, then system PM, then special
+#   installers (fzf-update, fisher, pipx). Always updates fisher plugins first.
+#
+# EXAMPLE
+#   _fish_deps_update
 function _fish_deps_update
     _fish_deps_catalog