examples: add AG2 multi-agent chat over private documents

[faridun-ag2]

Description

Adds an example demonstrating AG2 multi-agent orchestration on top of PrivateGPT. Three specialized agents collaborate to analyze private documents using PrivateGPT's RAG API as a tool.

AG2 is an open-source multi-agent framework with 500K+ monthly PyPI downloads, 4,300+ GitHub stars, and 400+ contributors.

Architecture

User question → AG2 GroupChat → PrivateGPT API (localhost:8001)
  ├── Researcher: RAG queries with use_context=true
  ├── Analyst: follow-up queries to verify findings
  └── Writer: synthesizes final answer with source references

What it demonstrates

• AG2 GroupChat with automatic speaker selection
• PrivateGPT's /v1/chat/completions as a registered AG2 tool
• Document-grounded multi-agent reasoning
• Privacy-preserving architecture: retrieval is 100% local, reasoning can be local or external

Why multi-agent + private RAG

• Single-query RAG can miss context. Multiple agents with different query strategies (broad search, targeted follow-ups) provide more thorough coverage.
• The Researcher/Analyst/Writer pattern mirrors real document analysis workflows.

Files

• examples/ag2_multiagent.py — standalone script, no changes to core PrivateGPT
• examples/README.md — setup and usage guide

Testing

• Script runs end-to-end with PrivateGPT server (mock mode)
• Researcher agent calls PrivateGPT RAG and receives document-grounded responses with sources
• Analyst agent asks follow-up queries for verification
• Writer agent synthesizes findings and terminates conversation
• Graceful error when PrivateGPT is not running
• No hardcoded secrets
• Passes Black, Ruff formatting

[faridun-ag2]

Hey @imgarylai @Toad882 @atari2600tim !

This PR adds a standalone AG2 multi-agent example (examples/ directory only, zero changes to core PrivateGPT). It's been tested end-to-end against PrivateGPT in mock mode with GPT-4o-mini for agent reasoning — full details in the PR description.

Note on the failing pre-commit CI check: The failure is not related to this PR. It's caused by a deprecated actions/upload-artifact@v3 in the repository's GitHub Actions workflow, which GitHub now auto-rejects:

Error: This request has been automatically failed because it uses a deprecated version of actions/upload-artifact: v3.

This affects all PRs on this repo. Upgrading to actions/upload-artifact@v4 in .github/workflows/ should fix it.

Would appreciate a review when you get a chance. Happy to adjust anything!