price-gate's EVAL_GATE_UNPRICED (and --allow-unpriced) now covers only no
plan declared at all or a plan with zero call lines. A declared price_file
missing from the HEAD commit is EVAL_PLAN_STALE (exit 1, not overridable); an
unresolvable origin/<base> is POLICY_SOURCE_UNAVAILABLE and an invalid
.gstack/pregate.json is PREGATE_CONFIG_INVALID (both exit 3).
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>