feat: add RAG-powered Document Q&A Agent (uAgents + LangChain + Gemini)

[davi713albano-coder]

Summary

Adds a RAG-powered Document Q&A Agent (contributors/rag-document-qa-agent/) that ingests PDF/plain-text documents, embeds them with HuggingFace sentence-transformers, stores vectors in ChromaDB, and answers natural-language questions via the uAgents Chat Protocol using Google Gemini 2.0 Flash as the LLM backbone.

Closes #128

What the agent does

1. Accepts a document (PDF, .txt, .md, .csv) via the ingest <path> command or DOCUMENT_PATH env var
2. Chunks the document with RecursiveCharacterTextSplitter and embeds with all-MiniLM-L6-v2
3. Stores vectors in a local ChromaDB instance
4. Answers questions grounded in document content via the Chat Protocol — no hallucinated information
5. Supports conversational follow-up (chat history window)

Commands

Command	Description
ingest <path>	Load a document into the vector store
status	Check if a document is currently loaded
<question>	Ask a question about the loaded document

Tech Stack

• uAgents — agent runtime + Chat Protocol
• LangChain — RAG pipeline orchestration
• Google Gemini 2.0 Flash — LLM (free tier, no cost barrier)
• ChromaDB — local vector store
• HuggingFace sentence-transformers (all-MiniLM-L6-v2) — embeddings
• pypdf — PDF parsing

Type of Change

• New agent example

Checklist

• I have starred this repository.
• New community agents are under contributors/<agent-name>/ (not repo root).
• I ran ruff check . — all checks passed.
• I ran ruff format . — all files formatted.
• I added/updated README.md for changed example(s).
• I added .env.example if environment variables are required.
• I added demo image/GIF (if applicable).
• I added agent profile link (if applicable).
• I updated contributors/CHANGELOG.md for community agent changes, or root CHANGELOG.md for other non-doc changes.
• I added my agent to the Community Contributors table in root README.md (if new agent).
• I verified paths/commands used in docs.
• I understand this PR requires maintainer review before merge (review-required CI).

Related Issue

Closes #128

Notes for Reviewers

• The demo.png is a placeholder — a real screenshot should be added after manual testing.
• The agent follows the same patterns as existing contributor agents (community_agent, news-summarizer-agent).
• Gemini 2.0 Flash free tier is used so there's no cost barrier for contributors testing this example.