fix: skip unresolved skill memory updates

[RerankerGuo]

Summary

• Fixes fix: dont create skill everytime #1769.
• Skip skill-memory updates whose old_memory_id cannot be resolved from recalled existing skills.
• Add a regression test for unresolved update filtering.

Validation

• python3 -m py_compile src/memos/mem_reader/read_skill_memory/process_skill_memory.py tests/mem_reader/test_skill_memory.py
• python3 -m pytest tests/mem_reader/test_skill_memory.py -q (blocked: No module named pytest)
• 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 after clearing the cloud workspace disk.

• Run: tr-40071e1e-0f1 on cloud test-engine 10010
• memos_python_core/changed-repo-python: 3 passed, 0 failed, 0 skipped

Manual code review is still required before merge.