feat: route composite cube writes by responsibility

[RerankerGuo]

Summary

• Fixes Improve responsibility-aware routing in CompositeCubeView #1705.
• Add explicit metadata-based routing for CompositeCubeView writes and feedback while preserving fan-out fallback.
• Preserve cube provenance on merged search results by filling missing cube_id values.
• Add multi-cube regression tests for target-cube routing, responsibility routing, fallback fan-out, and search provenance.

Validation

• python3 -m py_compile src/memos/multi_mem_cube/composite_cube.py tests/multi_mem_cube/test_composite_cube.py
• python3 -m pytest tests/multi_mem_cube/test_composite_cube.py -q (blocked: No module named pytest)
• PYTHONPATH=src python3 manual regression invocation (blocked: No module named pydantic)
• make format (blocked: poetry: Command not found)

[Memtensor-AI]

Automated Test Results: PASSED

Cloud test-engine full-audit rerun against dev-v2.0.22 completed successfully.

• Run: tr-85560456-4a3 on cloud test-engine 10011
• memos_python_core/changed-repo-python: 4 passed, 0 failed, 0 skipped

Manual code review is still required before merge.