fix(manager): validate flag tokens to prevent injection via config (#1410)

Address review feedback: sanitize manager.flags values to allow only
CLI-safe tokens (--flag patterns and alphanumeric values). Invalid
tokens are dropped with a stderr warning. Prevents prompt injection
via compromised config.json.

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

tests/init-manager.test.cjs

@@ -453,4 +453,30 @@ describe('init manager', () => {
     assert.strictEqual(output.manager_flags.plan, '--skip-research');
     assert.strictEqual(output.manager_flags.execute, '--interactive');
   });
+
+  test('sanitizes invalid manager_flags to prevent injection (#1410)', () => {
+    writeState(tmpDir);
+    writeRoadmap(tmpDir, [{ number: '1', name: 'Test' }]);
+
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({
+        manager: {
+          flags: {
+            discuss: '; rm -rf /',
+            plan: '--valid-flag',
+            execute: '$(whoami)',
+          }
+        }
+      })
+    );
+
+    const result = runGsdTools('init manager', tmpDir);
+    const output = JSON.parse(result.output);
+
+    // Invalid flags should be sanitized to empty string
+    assert.strictEqual(output.manager_flags.discuss, '', 'injection attempt should be sanitized');
+    assert.strictEqual(output.manager_flags.plan, '--valid-flag', 'valid flag should pass through');
+    assert.strictEqual(output.manager_flags.execute, '', 'command substitution should be sanitized');
+  });
 });