Fix local memory fallbacks and temporary topic scoring

[elinawang626-design]

Summary

This PR fixes several fallback inconsistencies around local memory and temporary-topic handling.

Changes

• unify proactive memu_memory with the shared executor so proactive mode and normal chat use the same fallback behavior
• require complete remote memU config before attempting remote retrieval
• fall back to local controlled memory when remote retrieve returns a non-OK response or throws
• fall back to local controlled memory when remote memorization crashes before creating a task
• fall back to heuristic temporary-topic scoring when no Claude API key is configured
• add regression tests for memory fallback behavior and heuristic temporary-topic transitions

Verification

• npm run typecheck
• npm test

[elinawang626-design]

Closing this in favor of #33, which contains the same bugfix scope rebased into a clean upstream-compatible PR with only the intended 6-file change set.