ci: add 30-minute Dagger-side timeout to Check pipeline

If any step hangs (stuck service, deadlocked test, network stall), the
pipeline will now cancel itself after 30 min rather than blocking the
runner indefinitely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ci/main.go

@@ -400,6 +400,9 @@ func (m *Ci) TestSyncReliability(ctx context.Context) (string, error) {
 
 // Check runs the full check suite.
 func (m *Ci) Check(ctx context.Context) (string, error) {
+	ctx, cancel := context.WithTimeout(ctx, 30*time.Minute)
+	defer cancel()
+
 	if _, err := m.CheckHygiene(ctx); err != nil {
 		return "Hygiene check failed", err
 	}