Add OSS adapter and sync adoption flows for LangChain and LangGraph

[arpitasarker01]

Summary

• add the local service, generic sync push/pull flow, and Postgres-ready receiver backend
• add LangGraph and LangChain adapters with runnable sync demos and self-host examples
• keep OSS local-first and separate from hosted-only features while improving the README adoption path

Verification

• /tmp/forkit_venv/bin/python -m pytest -v
• 190 passed