fix: improve long-task wrapper cli errors

scripts/test_long_task_governor_wrapper.mjs

@@ -1,6 +1,6 @@
 #!/usr/bin/env node
 import assert from 'node:assert/strict';
-import { execFileSync } from 'node:child_process';
+import { execFileSync, spawnSync } from 'node:child_process';
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 
@@ -102,8 +102,25 @@ function runFixture(fixture) {
   };
 }
 
+function assertErrorCase(name, args, expectedStderr, input) {
+  const result = spawnSync(process.execPath, [wrapperPath, ...args], {
+    cwd: repoRoot,
+    encoding: 'utf8',
+    input,
+  });
+
+  assert.notEqual(result.status, 0, `${name}: expected non-zero exit`);
+  assert.equal(result.stdout, '', `${name}: expected empty stdout`);
+  assert.equal(result.stderr.trim(), expectedStderr, `${name}: unexpected stderr`);
+}
+
 function main() {
   const results = fixtures.map(runFixture);
+
+  assertErrorCase('invalid json', ['--compact'], 'INVALID_JSON: input must be valid JSON', 'not-json\n');
+  assertErrorCase('missing input value', ['--input'], 'CLI_ERROR: --input requires a value');
+  assertErrorCase('unknown argument', ['--bogus'], 'CLI_ERROR: unknown argument: --bogus');
+
   const summary = {
     passed: results.length,
     fixtures: results.map(({ name, output }) => ({
@@ -114,6 +131,7 @@ function main() {
       requiredNextAction: output.requiredNextAction,
       handoffMode: output.handoff.mode,
     })),
+    errorCases: 3,
   };
 
   process.stdout.write(JSON.stringify(summary, null, 2) + '\n');