feat(memos-local-plugin): add lightweight memory mode

[hijzy]

Summary

• add configurable lightweight memory mode for memos-local-plugin capture/retrieval/orchestration
• move lightweight memory control to Settings > General and place password protection above danger actions
• keep lightweight trace data out of higher-level task/skill/policy/world-model views and add empty states
• tighten password-protected /api/v1/ping gating to match auth expectations

Tests

• npm test
• npm run lint
• npm run build
• npm run build:viewer
• npm pack --silent