feat: sync heartbeats, eval:trend --team, setup guide, 10 new tests

- 005_sync_heartbeats.sql migration for connectivity testing
- eval:trend --team flag pulls team eval data (graceful fallback)
- docs/TEAM_SYNC_SETUP.md step-by-step setup guide
- Design doc status updated to Phase 2 complete
- 10 new tests for sync show formatting functions

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

supabase/migrations/005_sync_heartbeats.sql

@@ -0,0 +1,25 @@
+-- 005_sync_heartbeats.sql — Lightweight table for sync connectivity tests.
+--
+-- Used by `gstack-sync test` to validate the full push/pull flow
+-- without polluting real data tables.
+
+create table if not exists sync_heartbeats (
+  id uuid primary key default gen_random_uuid(),
+  team_id uuid references teams(id) not null,
+  user_id uuid references auth.users(id),
+  hostname text not null default '',
+  timestamp timestamptz not null default now()
+);
+
+-- RLS
+alter table sync_heartbeats enable row level security;
+
+create policy "team_insert" on sync_heartbeats
+  for insert with check (
+    team_id in (select team_id from team_members where user_id = auth.uid())
+  );
+
+create policy "team_read" on sync_heartbeats
+  for select using (
+    team_id in (select team_id from team_members where user_id = auth.uid())
+  );