feat(shell): add __fish_variable_check utility function

Provides a reusable helper for evaluating configuration variable
truthiness. Returns 0 (true), 1 (false), 2 (empty/unset), or
3 (unrecognized value) — enables future strict-mode and opinionated-
components guards without repeating inline logic.

functions/__fish_variable_check.fish

@@ -0,0 +1,60 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   __fish_variable_check <variable_name>
+#
+# DESCRIPTION
+#   Evaluates a given variable's value to determine its truthiness or falsiness.
+#   It safely dereferences the variable name and performs case-insensitive matching.
+#
+# ARGUMENTS
+#   variable_name  The name of the variable to check (e.g., __fish_config_strict_mode)
+#                  Do NOT pass the value directly (do not use the $ prefix).
+#
+# RETURNS
+#   0  True    (Opt-In)  : Matches 1, true, yes, on, y
+#   1  False   (Opt-Out) : Matches 0, false, no, off, n
+#   2  Empty             : Variable is unset or contains an empty string
+#   3  Garbage           : Variable contains an unrecognized value
+#
+# EXAMPLE
+#   __fish_variable_check __fish_config_strict_mode
+#   if test $status -eq 0
+#       echo "Strict mode enabled!"
+#   end
+#
+function __fish_variable_check --description "Check if a variable is set to a truthy or falsy value."
+    set -l var_name $argv[1]
+
+    # Make sure they actually passed an argument
+    if test -z "$var_name"
+        return 2
+    end
+
+    # 2: Empty / Unset (Check if the variable exists in the environment at all)
+    if not set -q $var_name
+        return 2
+    end
+
+    # Dereference the variable name to get its actual value using $$
+    set -l val (string lower "$$var_name")
+
+    # 2: Empty / Unset (Check if it exists but is just an empty string)
+    if test -z "$val"
+        return 2
+    end
+
+    # 0: True (Opt-In)
+    if contains -- "$val" 1 true yes on y
+        return 0
+    end
+
+    # 1: False (Opt-Out)
+    if contains -- "$val" 0 false no off n
+        return 1
+    end
+
+    # 3: Garbage / Unrecognized
+    return 3
+end