feat: add ESLint with error-handling rules

Add ESLint v9.35+ with typescript-eslint recommended config and
error-handling rules: preserve-caught-error (enforces { cause } when
re-throwing), no-unused-vars with caughtErrors:all, and
eslint-plugin-no-catch-all (warns on catch blocks that don't rethrow).

Fix existing violations: add error cause to container-runtime rethrow,
prefix unused vars with underscore, remove unused imports.

https://claude.ai/code/session_01JPjzhBp9PR5LtfLWVDrYrH

src/container-runner.ts

@@ -698,7 +698,7 @@ export function writeGroupsSnapshot(
   groupFolder: string,
   isMain: boolean,
   groups: AvailableGroup[],
-  registeredJids: Set<string>,
+  _registeredJids: Set<string>,
 ): void {
   const groupIpcDir = resolveGroupIpcPath(groupFolder);
   fs.mkdirSync(groupIpcDir, { recursive: true });