auto-detect non-interactive mode when no terminal is present

chezmoi run_onchange scripts don't receive CLI arguments, so the
--non-interactive flag only works when running the script directly.
Now also checks if stdin/stdout are real terminals to auto-detect
non-interactive contexts like Vagrant provisioning.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

run_onchange_after_install_main_deps.sh.tmpl

@@ -63,6 +63,11 @@ while [ $# -gt 0 ]; do
   shift
 done
 
+# Auto-detect non-interactive mode when no real terminal is present
+if [ ! -t 0 ] || [ ! -t 1 ]; then
+  NON_INTERACTIVE=1
+fi
+
 # Timeout portability (macOS has gtimeout via coreutils, not timeout)
 TIMEOUT_BIN="$(command -v timeout || command -v gtimeout || true)"