chore: merge upstream v2.0.5, resolve conflicts, bump to v2.1.0

- Merged upstream/main (supermemoryai/opencode-supermemory v2.0.5)
- Resolved conflicts in package.json and src/index.ts
- Adopted upstream prt_ prefix ID format (removed custom generatePartId)
- Retained all max features: Japanese keywords, signal extraction,
  incremental capture, entity context, session auto-save, compaction hooks
- Version bumped to 2.1.0