fix: ACP test collection fails without acp extra; ssh mark unregistered (#779)

Resolves #779. Two issues fixed:

tests/acp/conftest.py (new):
- Module-level collect_ignore when acp is not installed
- Prevents pytest --collect-only from failing with ModuleNotFoundError
- 6 ACP test files cleanly skipped instead of erroring

tests/conftest.py:
- Registered ssh, integration, slow pytest markers
- Eliminates PytestUnknownMarkWarning

Before: 11,470 collected, 6 errors
After:  11,472 collected, 0 errors

tests/conftest.py

@@ -15,6 +15,12 @@ PROJECT_ROOT = Path(__file__).parent.parent
 if str(PROJECT_ROOT) not in sys.path:
     sys.path.insert(0, str(PROJECT_ROOT))
 
+# Register custom markers
+def pytest_configure(config):
+    config.addinivalue_line("markers", "ssh: marks tests requiring SSH connectivity")
+    config.addinivalue_line("markers", "integration: marks integration tests")
+    config.addinivalue_line("markers", "slow: marks slow tests")
+
 
 @pytest.fixture(autouse=True)
 def _isolate_hermes_home(tmp_path, monkeypatch):
@@ -119,3 +125,5 @@ def _enforce_test_timeout():
     yield
     signal.alarm(0)
     signal.signal(signal.SIGALRM, old)
+
+