feat: unified gstack eval CLI with list, compare, push, cache, cost

- lib/cli-eval.ts: routes to list/compare/summary/push/cost/cache/watch
  subcommands. Ports logic from 4 separate scripts into unified entry.
  Adds ANSI color for TTY (respects NO_COLOR), --limit flag for list.
- bin/gstack-eval: bash wrapper matching bin/gstack-sync pattern
- package.json: eval:* scripts now point to lib/cli-eval.ts
- supabase/migrations/004_eval_costs.sql: per-model cost tracking + RLS
- docs/eval-result-format.md: public format spec for any language
- test/lib-eval-cli.test.ts: integration tests (spawn CLI subprocess)
  including 3 push failure modes (file-not-found, invalid schema,
  sync unavailable)

215 tests passing across 13 files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-eval

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# gstack eval — unified eval CLI
+# Delegates to lib/cli-eval.ts via bun
+
+GSTACK_DIR="${GSTACK_DIR:-$(cd "$(dirname "$0")/.." && pwd)}"
+exec bun run "$GSTACK_DIR/lib/cli-eval.ts" "$@"