fix: remove unused short_name parameter from branch numbering functions

The check_existing_branches (bash) and Get-NextBranchNumber (PowerShell)
functions no longer use the short_name parameter since they now find the
global maximum across ALL features. This commit:

1. Removes the unused parameter from function signatures
2. Updates all call sites to not pass the parameter

This prevents the scripts from failing when the function is called with
the wrong number of arguments.

scripts/powershell/create-new-feature.ps1

@@ -210,7 +210,7 @@ if ($ShortName) {
 if ($Number -eq 0) {
     if ($hasGit) {
         # Check existing branches on remotes
-        $Number = Get-NextBranchNumber -ShortName $branchSuffix -SpecsDir $specsDir
+        $Number = Get-NextBranchNumber -SpecsDir $specsDir
     } else {
         # Fall back to local directory check
         $Number = (Get-HighestNumberFromSpecs -SpecsDir $specsDir) + 1