feat(history): add support for Nth history index in substitution

Extends the interactive history substitution function to support an
optional third field `old/new/index`.

- Keeps default behavior of targeting the most recent command (index 1).
- Allows targeting a specific history entry by appending a number.
- Validates that the index is a positive integer before attempting retrieval.
- Safely handles cases where the requested history index does not exist.

Example: 'cat/bat/5' now replaces 'cat' with 'bat' in the 5th previous command.

functions/__interactive_history_sub.fish

@@ -6,8 +6,18 @@ function __interactive_history_sub
         set -l parts (string split '/' -- "$current_line")
         set -l old $parts[1]
         set -l new $parts[2]
-        set -l expanded (string replace -a -- "$old" "$new" "$last_cmd")
+        set -l history_index 1
+        
+        if test (count $parts) -ge 3; and string match -qr '^[1-9][0-9]*$' -- "$parts[3]"
+            set history_index $parts[3]
+        end
+        
+        set -l target_cmd $history[$history_index]
+        
+        if test -n "$target_cmd"
+            set -l expanded (string replace -a -- "$old" "$new" "$target_cmd")
             commandline -r "$expanded"
+        end
     else
         if test -z "$current_line"
             commandline -r "sudo $last_cmd"