feat: add TunnelConfig dataclass and YAML I/O

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

tests/conftest.py

@@ -0,0 +1,19 @@
+import pytest
+from autossh_mgr.config import TunnelConfig, ensure_dirs
+
+
+@pytest.fixture
+def config_dir(tmp_path):
+    ensure_dirs(tmp_path)
+    return tmp_path
+
+
+@pytest.fixture
+def sample_tunnel():
+    return TunnelConfig(
+        name="web-service",
+        host="relay.example.com",
+        user="deploy",
+        local_port=8080,
+        remote_port=18080,
+    )