feat(memory): add optional OpenViking agent middleware

[NikkorW]

Summary

• add an optional OpenViking middleware that recalls bounded long-term context before agent execution
• capture user/assistant messages and bounded, redacted tool activity, then commit the session after the run
• keep the integration fail-open and separate from Mem0 and SpoonOS checkpoint/runtime state
• add the optional SDK dependency, mocked tests, and local/remote setup documentation

Testing

• uvx ruff check spoon_ai/middleware/openviking_memory.py tests/test_openviking_memory_middleware.py examples/openviking_memory_middleware_demo.py
• uv run --with pytest --with pytest-asyncio pytest -q tests/test_openviking_memory_middleware.py tests/test_prompt_caching.py (15 passed)
• uv run --python 3.13 --with build python -m build
• uv run --with twine twine check dist/*
• git diff --check
• verified OpenVikingMemoryMiddleware constructs with openviking-sdk==0.1.6 through the optional dependency
• OpenViking embedded integration at upstream commit eda69f9: a payload generated by this middleware successfully completed session creation, batch ingestion, context readback with ToolPart validation, session commit/archive, and search using OpenViking's real Session/storage implementation with fake model providers

Addresses #283