Harden memu retrieval and topic scoring fallbacks

[elinawang626-design]

Summary

This PR tightens two fallback paths that currently fail silently or behave inconsistently on main.

Changes

• route proactive memu_memory through the shared memu executor so proactive mode and normal chat use the same retrieval path
• require complete memU configuration before attempting remote retrieval
• return explicit errors for non-OK memU retrieval responses instead of treating them as successful results
• fall back to heuristic temporary-topic scoring when no Claude API key is configured
• add regression tests for memU retrieval error handling and heuristic temporary-topic transitions

Verification

• npx vitest run src/main/tools/__tests__/memu.executor.test.ts src/main/services/agent/context/layered/__tests__/temporary-topic.test.ts

Notes

• I also ran broader repo checks locally and saw existing unrelated failures in the current upstream codebase (for example in secure-storage and several typecheck errors), so this PR keeps verification focused on the changed paths.