feat: add self-update command and bump to 2.0.1

- Add `coinhunter update` CLI command for pipx/pip upgrade
- README: document update behavior and recommend pipx install
- Dynamic version badge with cacheSeconds=60
- Version bump: 2.0.0 → 2.0.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_cli.py

@@ -36,3 +36,12 @@ class CLITestCase(unittest.TestCase):
             result = cli.main(["market", "tickers", "BTCUSDT"])
         self.assertEqual(result, 1)
         self.assertIn("error: boom", stderr.getvalue())
+
+    def test_update_dispatches(self):
+        captured = {}
+        with patch.object(cli, "self_update", return_value={"command": "pipx upgrade coinhunter", "returncode": 0}), patch.object(
+            cli, "print_json", side_effect=lambda payload: captured.setdefault("payload", payload)
+        ):
+            result = cli.main(["update"])
+        self.assertEqual(result, 0)
+        self.assertEqual(captured["payload"]["returncode"], 0)