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/remote-control.test.ts

@@ -37,7 +37,7 @@ describe('remote-control', () => {
   let readFileSyncSpy: ReturnType<typeof vi.spyOn>;
   let writeFileSyncSpy: ReturnType<typeof vi.spyOn>;
   let unlinkSyncSpy: ReturnType<typeof vi.spyOn>;
-  let mkdirSyncSpy: ReturnType<typeof vi.spyOn>;
+  let _mkdirSyncSpy: ReturnType<typeof vi.spyOn>;
   let openSyncSpy: ReturnType<typeof vi.spyOn>;
   let closeSyncSpy: ReturnType<typeof vi.spyOn>;
 
@@ -50,7 +50,7 @@ describe('remote-control', () => {
     stdoutFileContent = '';
 
     // Default fs mocks
-    mkdirSyncSpy = vi
+    _mkdirSyncSpy = vi
       .spyOn(fs, 'mkdirSync')
       .mockImplementation(() => undefined as any);
     writeFileSyncSpy = vi