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-runtime.ts

@@ -96,7 +96,7 @@ export function ensureContainerRuntimeRunning(): void {
     console.error(
       '╚════════════════════════════════════════════════════════════════╝\n',
     );
-    throw new Error('Container runtime is required but failed to start');
+    throw new Error('Container runtime is required but failed to start', { cause: err });
   }
 }