feat(mcp,acp): Repository.WithTx + acp.Repository.InTx

Cross-module transactional support for spec §6.1: ACP CreateSession must
atomically insert acp_sessions row + mcp_tokens row (FK NOT NULL).

- mcp.Repository.WithTx(tx) -> Repository (tx-bound, no pool)
- acp.Repository.InTx(ctx, fn) -> error (pgx.BeginFunc wrapper)
- acp.Repository.WithTx(tx) -> Repository (tx-bound)
- Update fakeRepos in test files to satisfy new interface methods

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

internal/mcp/token_service_test.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -122,6 +123,10 @@ func (r *fakeRepo) DeleteByID(ctx context.Context, id uuid.UUID) error {
 	return nil
 }
 
+func (r *fakeRepo) WithTx(tx pgx.Tx) mcp.Repository {
+	return r
+}
+
 // ===== fake audit =====
 
 type fakeAudit struct {