Default MCP tab tools to the active tab

Requiring an explicit tab_id forced every navigate or close through a
preceding tabs_list call, which costs an MCP client a full round trip just to
learn the ID the browser already considers current.

navigate and tabs_close now resolve the active tab when tab_id is omitted,
matching the screenshot tool. Resolution is explicit rather than forwarding
None into the SDK, so the acting tool knows which tab it touched; tabs_close
reports it, since closing the wrong tab is not recoverable.

This stays in the MCP layer: the SDK and CLI signatures are unchanged.

extension/test/jobs.test.ts

@@ -1,7 +1,7 @@
 // @ts-nocheck
 import { test, mock } from 'node:test';
 import assert from 'node:assert/strict';
-import { JobManager, JOB_TIMEOUT_MS, MAX_FINISHED_JOBS, pruneFinishedJobs } from '../src/classes/JobManager';
+import { JobManager, JOB_TIMEOUT_MS, MAX_FINISHED_JOBS, MAX_RUNNING_JOBS, pruneFinishedJobs } from '../src/classes/JobManager';
 import { makeChromeMock } from './chrome-mock';
 
 // Drain pending microtasks (finalize() chains several awaits). setImmediate is
@@ -129,6 +129,21 @@ test('JobManager: a runner that settles after the watchdog cannot resurrect the
   mock.timers.reset();
 });
 
+test('JobManager: rejects new background jobs above the running-job cap', async () => {
+  mock.timers.enable({ apis: ['setInterval', 'setTimeout'] });
+  globalThis.chrome = makeChromeMock();
+  const mgr = new JobManager();
+  for (let i = 0; i < MAX_RUNNING_JOBS; i++) {
+    await mgr.start(`running${i}`, {}, () => new Promise(() => {}));
+  }
+  await assert.rejects(
+    () => mgr.start('overflow', {}, async () => 'nope'),
+    /too many background jobs running/,
+  );
+  assert.equal(mgr.list().filter(job => job.status === 'running').length, MAX_RUNNING_JOBS);
+  mock.timers.reset();
+});
+
 test('JobManager: persisted set keeps running jobs even past the finished cap', async () => {
   mock.timers.enable({ apis: ['setInterval', 'setTimeout'] });
   globalThis.chrome = makeChromeMock();