fix: move input validation before collision check to cover all paths

- Bash: reject non-numeric and zero --number before if/elif (fixes
  crash with --allow-existing-branch + non-numeric --number)
- PowerShell: reject negative -Number before if/elseif (same issue)
- Ensures consistent behavior: 0 is rejected in Bash (PS already
  treats 0 as auto-detect)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: fsilvaortiz

scripts/bash/create-new-feature.sh

@@ -249,6 +249,14 @@ if [ "$USE_TIMESTAMP" = true ]; then
     FEATURE_NUM=$(date +%Y%m%d-%H%M%S)
     BRANCH_NAME="${FEATURE_NUM}-${BRANCH_SUFFIX}"
 else
+    # Validate --number input when provided
+    if [ -n "$BRANCH_NUMBER" ]; then
+        if ! echo "$BRANCH_NUMBER" | grep -q '^[0-9]\+$' || [ "$((10#$BRANCH_NUMBER))" -lt 1 ]; then
+            >&2 echo "Error: --number requires a positive integer, got '$BRANCH_NUMBER'"
+            exit 1
+        fi
+    fi
+
     # Determine branch number
     if [ -z "$BRANCH_NUMBER" ]; then
         # No manual number provided -- auto-detect
@@ -263,10 +271,6 @@ else
     elif [ "$ALLOW_EXISTING" = false ]; then
         # Manual number provided -- validate it is not already in use
         # (skip when --allow-existing-branch, as the caller intentionally targets an existing branch)
-        if ! echo "$BRANCH_NUMBER" | grep -q '^[0-9]\+$'; then
-            >&2 echo "Error: --number requires a positive integer, got '$BRANCH_NUMBER'"
-            exit 1
-        fi
         MANUAL_NUM=$((10#$BRANCH_NUMBER))
         MANUAL_NUM_PADDED=$(printf "%03d" "$MANUAL_NUM")
         NUMBER_IN_USE=false

scripts/powershell/create-new-feature.ps1

@@ -201,6 +201,12 @@ if ($Timestamp) {
     $featureNum = Get-Date -Format 'yyyyMMdd-HHmmss'
     $branchName = "$featureNum-$branchSuffix"
 } else {
+    # Validate -Number input when provided
+    if ($Number -lt 0) {
+        Write-Error "-Number requires a positive integer, got $Number"
+        exit 1
+    }
+
     # Determine branch number
     if ($Number -eq 0) {
         # No manual number provided -- auto-detect
@@ -214,10 +220,6 @@ if ($Timestamp) {
     } elseif (-not $AllowExistingBranch) {
         # Manual number provided -- validate it is not already in use
         # (skip when -AllowExistingBranch, as the caller intentionally targets an existing branch)
-        if ($Number -lt 1) {
-            Write-Error "-Number requires a positive integer, got $Number"
-            exit 1
-        }
         $manualNumPadded = '{0:000}' -f $Number
         $numberInUse = $false