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/gip6.fish

@@ -1,7 +1,19 @@
 # Copyright (C) 2026 Rootiest
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
-# Get public IPv6 address
+# SYNOPSIS
+#   gip6
+#
+# DESCRIPTION
+#   Fetches and prints the machine's public IPv6 address using icanhazip.com.
+#   Prints an error message if IPv6 is unavailable on the current network.
+#
+# RETURNS
+#   0  IPv6 address printed
+#   1  IPv6 unavailable or not supported on this network
+#
+# EXAMPLE
+#   gip6
 function gip6 --description 'Get public IPv6 address'
     # Use -6 to force IPv6 and --fail to catch network errors
     set -l ip (curl -6 -s --fail https://icanhazip.com 2>/dev/null)