feat: comply with NO_COLOR spec and centralize terraform env checks

• colors.mk: ifndef NO_COLOR replaces ifneq ($(NO_COLOR),1) — spec requires any non-empty value to disable colors, not just "1"
• scripts: same fix — use -n test instead of = "1" comparison
• terraform.mk: move ENV/REGION defaults out of main.mk; inject them into REQUIRED_ENV_VARS only when terraform module is included; centralize check-env-vars to get-terraform-container-command so individual tf/* targets don't each declare it
• tests/colors: expand coverage for NO_COLOR=true, empty NO_COLOR, print helpers with and without colors
• .gitignore: add .envrc, reorder entries