Fix OpenHands skill loading with current SDK

[Rohithmatham12]

Summary

• load OpenHands skills from the current openhands.sdk.skills module
• keep a fallback to openhands.sdk.context.skills for older SDKs
• add regression coverage for the current SDK module path so evoskill run / evoskill eval no longer fail before agent execution

Fixes #44.

Testing

• .venv/bin/python -m pytest tests/test_openhands_runtime.py -q
• .venv/bin/python -m pytest tests/test_openhands_runtime.py tests/test_openhands_auth.py tests/test_openhands_contract.py -q
• git diff --check