chore: lock the plugin boundary and remove 2.9.0 leftovers

src/backend/plugins/migrations.ts

@@ -57,10 +57,7 @@ export function checksum(contents: string): string {
   return crypto.createHash("sha256").update(normalised).digest("hex");
 }
 
-export function migrationsDir(
-  pluginDir: string,
-  dialect: DatabaseDialect,
-): string {
+function migrationsDir(pluginDir: string, dialect: DatabaseDialect): string {
   return path.join(pluginDir, "migrations", dialect);
 }
 
@@ -211,11 +208,6 @@ export async function applyPluginMigrations(
   return pending.map((migration) => migration.id);
 }
 
-/** The dialect directories a plugin is expected to ship. */
-export function expectedDialects(): readonly DatabaseDialect[] {
-  return DIALECTS;
-}
-
 /**
  * Renames a legacy core table into a plugin's namespace, or creates it fresh.
  *