Document LightRAG and RAG-Anything upstream pulls

[1neilkou]

Motivation

• Track the requested upstream RAG projects so they can be pulled into this workspace and to record the intended target branches.

Description

• Add README.md with git clone/git -C ... pull --ff-only instructions for LightRAG and RAG-Anything, and add external_repos.lock that lists the upstream repository URLs and the main branch.

Testing

• Ran git ls-remote --heads https://github.com/HKUDS/LightRAG.git and git ls-remote --heads https://github.com/HKUDS/RAG-Anything.git which were blocked by the environment proxy returning HTTP 403, and git status --short --branch which completed successfully.

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Codex Task