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>

supabase/migrations/004_eval_costs.sql

@@ -0,0 +1,39 @@
+-- Per-model cost tracking for eval runs.
+-- Stores cost breakdown by model so teams can analyze spend patterns.
+
+create table eval_costs (
+  id uuid primary key default gen_random_uuid(),
+  team_id uuid references teams(id) not null,
+  eval_run_id uuid references eval_runs(id) on delete cascade,
+  model text not null,
+  calls int not null,
+  input_tokens int not null,
+  output_tokens int not null,
+  estimated_cost_usd numeric(10,6) not null,
+  created_at timestamptz default now()
+);
+
+-- Index for querying costs by team and eval run
+create index idx_eval_costs_team_run on eval_costs(team_id, eval_run_id);
+
+-- RLS: team members can read/insert their team's costs
+alter table eval_costs enable row level security;
+
+create policy "Team members can read costs"
+  on eval_costs for select
+  using (team_id in (
+    select team_id from team_members where user_id = auth.uid()
+  ));
+
+create policy "Team members can insert costs"
+  on eval_costs for insert
+  with check (team_id in (
+    select team_id from team_members where user_id = auth.uid()
+  ));
+
+create policy "Admins can delete costs"
+  on eval_costs for delete
+  using (team_id in (
+    select team_id from team_members
+    where user_id = auth.uid() and role = 'admin'
+  ));