refactor: remove dead strategy prompt example branch

[RerankerGuo]

Summary

• Fixes refactor: remove dead remove_prompt_example branch in StrategyStructMemReader #1792.
• Remove the no-op remove_prompt_example branch from StrategyStructMemReader.
• Drop the now-unused strategy prompt example imports and dictionary entries.
• Add a static regression test to keep the dead branch from returning.

Validation

• static strategy_struct dead-branch test passed
• python3 -m py_compile src/memos/mem_reader/strategy_struct.py tests/mem_reader/test_strategy_struct_dead_branch.py
• python3 -m pytest tests/mem_reader/test_strategy_struct_dead_branch.py -q (blocked: No module named pytest)
• make format (blocked: poetry: Command not found)

Note

• There is an older open PR for this issue (refactor(mem-reader): remove dead remove_prompt_example branch in StrategyStructMemReader #1794), but it targets dev-v2.0.22 and is behind. This PR is a fresh minimal version based on main.

[Memtensor-AI]

Automated Test Results: PASSED

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

• Run: tr-77eaab06-165 on cloud test-engine 10011
• memos_python_core/changed-repo-python: 1 passed, 0 failed, 0 skipped

Manual code review is still required before merge.