serialize real-pql tests to fix parallel db contention

The pql integration tests (test/pql/client_test.dart) and pql.* daemon
handler tests (test/daemon/pql_commands_test.dart) each spawn a real
`pql` process against the shared on-disk .pql/pql.db. In the parallel
test pool, concurrent invocations contend for the SQLite lock and flake
with PqlException(69) (db busy) — surfaced reliably by the pql 1.10
record_id migration. They pass one-at-a-time, so isolation is the fix.

Tag both files @Tags(['serial']) and add a --concurrency=1 serial pass
to ci/test_core.sh (pty + serial together), mirroring ci/test.sh's
existing serial handling. The error-path companion uses a fake binary,
so it stays parallel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/daemon/pql_commands_test.dart

@@ -1,3 +1,15 @@
+/// Drives the pql.* daemon handlers against the real `pql` binary and the
+/// working directory's vault.
+///
+/// Tagged `serial`: each handler spawns a real `pql` process that opens the
+/// shared on-disk `.pql/pql.db`. Run in the parallel pool these contend for
+/// the SQLite lock and flake (`PqlException(69)`, db busy); one-at-a-time they
+/// pass — isolation is the real fix (T-193). The error-path companion
+/// (`pql_commands_errors_test.dart`) points at a fake binary, so it needs no
+/// such tag.
+@Tags(['serial'])
+library;
+
 import 'dart:io';
 
 import 'package:clide/clide.dart';