fix(workstream): require name arg for set, add --clear flag (#1527)

workstream set with no argument silently cleared the active workstream,
a footgun for users who forgot the name. Now requires a name arg and
errors with usage hint. Explicit clearing via --clear flag, which also
reports the previous workstream in its output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/workstream.test.cjs

@@ -268,6 +268,24 @@ describe('workstream set/get', () => {
     assert.ok(result.success);
     assert.strictEqual(result.output, 'ws-a');
   });
+
+  test('errors when set called with no name (#1527)', () => {
+    const result = runGsdTools(['workstream', 'set', '--raw'], tmpDir);
+    assert.ok(!result.success, 'should fail when no name provided');
+    assert.ok(result.error.includes('name required'), 'error should mention name required');
+  });
+
+  test('--clear explicitly unsets active workstream', () => {
+    // First set one
+    runGsdTools(['workstream', 'set', 'ws-b', '--raw'], tmpDir);
+    // Then clear
+    const result = runGsdTools(['workstream', 'set', '--clear', '--raw'], tmpDir);
+    assert.ok(result.success);
+    const data = JSON.parse(result.output);
+    assert.strictEqual(data.active, null);
+    assert.strictEqual(data.cleared, true);
+    assert.strictEqual(data.previous, 'ws-b');
+  });
 });
 
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