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/acp/agentkind_service_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -124,6 +125,14 @@ func (f *fakeAgentKindRepo) PurgeEventsBefore(context.Context, time.Time) (int64
 	panic("n/a")
 }
 
+func (f *fakeAgentKindRepo) InTx(context.Context, func(context.Context, pgx.Tx) error) error {
+	panic("n/a")
+}
+
+func (f *fakeAgentKindRepo) WithTx(pgx.Tx) acp.Repository {
+	return f
+}
+
 // agentKindRecordingRecorder：accumulate audit entries
 type agentKindRecordingRecorder struct{ entries []audit.Entry }