feat(scripts): add --allow-existing-branch flag to create-new-feature (#1999)

* feat(scripts): add --allow-existing-branch flag to create-new-feature

Add an --allow-existing-branch / -AllowExistingBranch flag to both
bash and PowerShell create-new-feature scripts. When the target branch
already exists, the script switches to it instead of failing. The spec
directory and template are still created if missing, but existing
spec.md files are not overwritten (prevents data loss on re-runs).

The flag is opt-in, so existing behavior is completely unchanged
without it. This enables worktree-based workflows and CI/CD pipelines
that create branches externally before running speckit.specify.

Relates to #1931. Also addresses #1680, #841, #1921.

Assisted-By: 🤖 Claude Code

* fix: address PR review feedback for allow-existing-branch

- Make checkout failure fatal instead of suppressing with || true (bash)
- Check $LASTEXITCODE after git checkout in PowerShell
- Use Test-Path -PathType Leaf for spec file existence check (PS)
- Add PowerShell static assertion test for -AllowExistingBranch flag

Assisted-By: 🤖 Claude Code

scripts/bash/create-new-feature.sh

@@ -3,6 +3,7 @@
 set -e
 
 JSON_MODE=false
+ALLOW_EXISTING=false
 SHORT_NAME=""
 BRANCH_NUMBER=""
 USE_TIMESTAMP=false
@@ -14,6 +15,9 @@ while [ $i -le $# ]; do
         --json)
             JSON_MODE=true
             ;;
+        --allow-existing-branch)
+            ALLOW_EXISTING=true
+            ;;
         --short-name)
             if [ $((i + 1)) -gt $# ]; then
                 echo 'Error: --short-name requires a value' >&2
@@ -45,10 +49,11 @@ while [ $i -le $# ]; do
             USE_TIMESTAMP=true
             ;;
         --help|-h)
-            echo "Usage: $0 [--json] [--short-name <name>] [--number N] [--timestamp] <feature_description>"
+            echo "Usage: $0 [--json] [--allow-existing-branch] [--short-name <name>] [--number N] [--timestamp] <feature_description>"
             echo ""
             echo "Options:"
             echo "  --json              Output in JSON format"
+            echo "  --allow-existing-branch  Switch to branch if it already exists instead of failing"
             echo "  --short-name <name> Provide a custom short name (2-4 words) for the branch"
             echo "  --number N          Specify branch number manually (overrides auto-detection)"
             echo "  --timestamp         Use timestamp prefix (YYYYMMDD-HHMMSS) instead of sequential numbering"
@@ -69,7 +74,7 @@ done
 
 FEATURE_DESCRIPTION="${ARGS[*]}"
 if [ -z "$FEATURE_DESCRIPTION" ]; then
-    echo "Usage: $0 [--json] [--short-name <name>] [--number N] [--timestamp] <feature_description>" >&2
+    echo "Usage: $0 [--json] [--allow-existing-branch] [--short-name <name>] [--number N] [--timestamp] <feature_description>" >&2
     exit 1
 fi
 
@@ -287,12 +292,19 @@ if [ "$HAS_GIT" = true ]; then
     if ! git checkout -b "$BRANCH_NAME" 2>/dev/null; then
         # Check if branch already exists
         if git branch --list "$BRANCH_NAME" | grep -q .; then
-            if [ "$USE_TIMESTAMP" = true ]; then
+            if [ "$ALLOW_EXISTING" = true ]; then
+                # Switch to the existing branch instead of failing
+                if ! git checkout "$BRANCH_NAME" 2>/dev/null; then
+                    >&2 echo "Error: Failed to switch to existing branch '$BRANCH_NAME'. Please resolve any local changes or conflicts and try again."
+                    exit 1
+                fi
+            elif [ "$USE_TIMESTAMP" = true ]; then
                 >&2 echo "Error: Branch '$BRANCH_NAME' already exists. Rerun to get a new timestamp or use a different --short-name."
+                exit 1
             else
                 >&2 echo "Error: Branch '$BRANCH_NAME' already exists. Please use a different feature name or specify a different number with --number."
+                exit 1
             fi
-            exit 1
         else
             >&2 echo "Error: Failed to create git branch '$BRANCH_NAME'. Please check your git configuration and try again."
             exit 1
@@ -305,13 +317,15 @@ fi
 FEATURE_DIR="$SPECS_DIR/$BRANCH_NAME"
 mkdir -p "$FEATURE_DIR"
 
-TEMPLATE=$(resolve_template "spec-template" "$REPO_ROOT") || true
 SPEC_FILE="$FEATURE_DIR/spec.md"
-if [ -n "$TEMPLATE" ] && [ -f "$TEMPLATE" ]; then
-    cp "$TEMPLATE" "$SPEC_FILE"
-else
-    echo "Warning: Spec template not found; created empty spec file" >&2
-    touch "$SPEC_FILE"
+if [ ! -f "$SPEC_FILE" ]; then
+    TEMPLATE=$(resolve_template "spec-template" "$REPO_ROOT") || true
+    if [ -n "$TEMPLATE" ] && [ -f "$TEMPLATE" ]; then
+        cp "$TEMPLATE" "$SPEC_FILE"
+    else
+        echo "Warning: Spec template not found; created empty spec file" >&2
+        touch "$SPEC_FILE"
+    fi
 fi
 
 # Inform the user how to persist the feature variable in their own shell