fix(scripts): suppress stdout from git fetch in create-new-feature.sh (#1876)

In multi-remote environments, `git fetch --all` outputs messages like
"Fetching origin" to stdout. Since `check_existing_branches()` only
redirected stderr (`2>/dev/null`), the stdout output was captured by
the `$(...)` command substitution calling this function, contaminating
the branch number return value and causing arithmetic errors like
`$((10#Fetching...))`.

Fix: redirect both stdout and stderr to /dev/null (`>/dev/null 2>&1`).

Author: seiya-koji

scripts/bash/create-new-feature.sh

@@ -138,7 +138,7 @@ check_existing_branches() {
     local specs_dir="$1"
 
     # Fetch all remotes to get latest branch info (suppress errors if no remotes)
-    git fetch --all --prune 2>/dev/null || true
+    git fetch --all --prune >/dev/null 2>&1 || true
 
     # Get highest number from ALL branches (not just matching short name)
     local highest_branch=$(get_highest_from_branches)