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.test.ts

@@ -40,7 +40,7 @@ describe('GroupQueue', () => {
     let concurrentCount = 0;
     let maxConcurrent = 0;
 
-    const processMessages = vi.fn(async (groupJid: string) => {
+    const processMessages = vi.fn(async (_groupJid: string) => {
       concurrentCount++;
       maxConcurrent = Math.max(maxConcurrent, concurrentCount);
       // Simulate async work
@@ -69,7 +69,7 @@ describe('GroupQueue', () => {
     let maxActive = 0;
     const completionCallbacks: Array<() => void> = [];
 
-    const processMessages = vi.fn(async (groupJid: string) => {
+    const processMessages = vi.fn(async (_groupJid: string) => {
       activeCount++;
       maxActive = Math.max(maxActive, activeCount);
       await new Promise<void>((resolve) => completionCallbacks.push(resolve));
@@ -104,7 +104,7 @@ describe('GroupQueue', () => {
     const executionOrder: string[] = [];
     let resolveFirst: () => void;
 
-    const processMessages = vi.fn(async (groupJid: string) => {
+    const processMessages = vi.fn(async (_groupJid: string) => {
       if (executionOrder.length === 0) {
         // First call: block until we release it
         await new Promise<void>((resolve) => {