feat: add migration runner with structured failure diagnostic (E00-S03-T05)

MigrationRunner applies pending migrations through the migration ledger
exactly once; when a migration fails it throws a MigrationFailedError
whose diagnostic is a structured object identifying the failing migration
(version), the failure phase (apply/record), the underlying cause, and the
applied/pending ledger state, serializable via toJSON. Re-exported from
the driver boundary so no other package needs the pg driver to run
migrations. Advisory lock (T04) and ready gate (T06) remain out of scope.

packages/database-postgres/src/index.ts

@@ -9,10 +9,11 @@
  *
  * This module is the driver boundary: it imports the PostgreSQL driver (`pg`)
  * and Kysely and re-exports the pieces the adapter is built on — the driver
- * surface (E00-S03-T02), the migration ledger (E00-S03-T03) and the migration
- * advisory lock (E00-S03-T04). The failure diagnostic (E00-S03-T05) lands in
- * a later story; until then the re-exports keep the driver reachable only
- * from here — the isolation is real, not a placeholder.
+ * surface (E00-S03-T02), the migration ledger (E00-S03-T03), the migration
+ * advisory lock (E00-S03-T04) and the migration runner with its failure
+ * diagnostic (E00-S03-T05). Keeping every re-export here means the driver
+ * stays reachable only from this package — the isolation is real, not a
+ * placeholder.
  */
 
 import { Pool } from 'pg';
@@ -21,3 +22,5 @@ import { Kysely, PostgresDialect } from 'kysely';
 export { Pool, Kysely, PostgresDialect };
 export { MigrationLedger, MIGRATION_LEDGER_TABLE } from './ledger.js';
 export { MigrationLock, MIGRATION_LOCK_KEY } from './lock.js';
+export { MigrationRunner, MigrationFailedError } from './runner.js';
+export type { Migration, MigrationDiagnostic, MigrationRunResult, MigrationFailurePhase } from './runner.js';