feat(app): add UnitOfWork and owner-bound repositories

Give each document command one postgres transaction and only let table owners unwrap the scope, so cross-owner writes can roll back together.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/freedoc/repository.go

@@ -0,0 +1,18 @@
+package freedoc
+
+import (
+	"context"
+	"fmt"
+
+	"chat-agent-system/internal/app/uow"
+)
+
+// Repository is a M3 stub. M1 freeform projection stays on the existing service path.
+type Repository struct{}
+
+func (Repository) ForbiddenInM1(ctx context.Context, scope uow.TxScope) error {
+	if _, err := uow.AsPG(scope); err != nil {
+		return err
+	}
+	return fmt.Errorf("freedoc: repository writes are not enabled until M3")
+}