fix: quiet git checkout in JSON mode and clarify registry check logic

1. Use `git checkout -q -b` with stdout redirect to prevent status
   messages from contaminating JSON output in create-new-feature.sh
2. Clarify specify.md registry check: only skip branching when
   extensions.git.enabled is present and explicitly false; proceed
   normally in all other cases (missing registry, missing entry, etc.)

Co-authored-by: mnriem <15701806+mnriem@users.noreply.github.com>
Agent-Logs-Url: https://github.com/github/spec-kit/sessions/67a6414f-e8ce-4455-95e5-299c72d86624

Author: Copilot

extensions/git/scripts/bash/create-new-feature.sh

@@ -343,7 +343,7 @@ if [ ${#BRANCH_NAME} -gt $MAX_BRANCH_LENGTH ]; then
 fi
 
 if [ "$HAS_GIT" = true ]; then
-    if ! git checkout -b "$BRANCH_NAME" 2>/dev/null; then
+    if ! git checkout -q -b "$BRANCH_NAME" >/dev/null 2>&1; then
         # Check if branch already exists
         if git branch --list "$BRANCH_NAME" | grep -q .; then
             if [ "$USE_TIMESTAMP" = true ]; then

templates/commands/specify.md

@@ -77,7 +77,8 @@ Given that feature description, do this:
 
    **Git extension check**: Before running the branch creation script, check if the git extension is enabled:
    - Check if `.specify/extensions/.registry` exists (a single JSON file tracking all extensions)
-   - If it exists, read the JSON and look for `extensions.git`; verify its `"enabled"` field is `true` (or not explicitly `false`)
+   - If it exists, read the JSON and look for an `extensions.git` entry
+   - **Only skip branching** when `extensions.git.enabled` is **present and explicitly `false`**. In all other cases — including when the registry file is missing, when `extensions.git` has no entry, or when `"enabled"` is `true` or absent — proceed with normal branch creation (extension script if present, else core `{SCRIPT}` fallback)
    - If the git extension is **disabled** (explicitly `"enabled": false`), **skip branch creation entirely** — do **not** run the branch creation script. Instead:
      - Derive a spec directory name from the short name, e.g. `specs/<short-name>/`
      - Explicitly set the following variables so later steps can use them: