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/group-queue.ts

@@ -351,7 +351,7 @@ export class GroupQueue {
     // via idle timeout or container timeout. The --rm flag cleans them up on exit.
     // This prevents WhatsApp reconnection restarts from killing working agents.
     const activeContainers: string[] = [];
-    for (const [jid, state] of this.groups) {
+    for (const [_jid, state] of this.groups) {
       if (state.process && !state.process.killed && state.containerName) {
         activeContainers.push(state.containerName);
       }