Reuse unchanged community summaries

[junyangyan4-svg]

Summary

• compute a stable member signature for each detected community
• reuse existing summaries and embeddings when the member set is unchanged
• backfill and index the new signature field in gm_communities

Why

This avoids regenerating the same community summary when a community has not materially changed.

Split from #39.

Verification

• npm test
• npx tsc --noEmit

[adoresever]

核心方案已采纳并合并：复用未变化社区的摘要能够明显减少重复 LLM 调用。合并时补充修正了社区 ID 变化后的摘要复用顺序，并加入迁移/回归测试。感谢贡献。