refactor(path): consolidate x-path-{append,prepend,remove} as thin wrappers

Add source guard to x-path so its functions can be loaded without
executing the main logic. Rewrite standalone path scripts to source
x-path and call the appropriate function directly, eliminating code
duplication while preserving source-ability for shell integration.

local/bin/x-path

@@ -227,6 +227,9 @@ do_check()
   fi
 }
 
+# If sourced, provide functions without executing main logic
+(return 0 2> /dev/null) && return
+
 #######################################
 # Main routine: Parse subcommand and arguments, normalize PATH,
 # and dispatch to the appropriate functionality.