Add agent config revisions, issue-approval links, and robust migration reconciliation

Add agent_config_revisions table for tracking agent configuration changes with
rollback support. Add issue_approvals junction table linking issues to approvals.
New migrations (0005, 0006) for permissions column and new tables. Rework migration
client with statement-level idempotency checks (table/column/index/constraint
existence) so migrations can be safely retried against partially-migrated databases.

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

packages/db/src/index.ts

@@ -3,7 +3,9 @@ export {
   ensurePostgresDatabase,
   inspectMigrations,
   applyPendingMigrations,
+  reconcilePendingMigrationHistory,
   type MigrationState,
+  type MigrationHistoryReconcileResult,
   migratePostgresIfEmpty,
   type MigrationBootstrapResult,
   type Db,