Add AG2 (formerly AutoGen) multi-agent research team demo with browser UI

[VasiliyRad]

Adds ag2/ — a single-container multi-agent research assistant using AG2 v0.11+ with a Gradio browser chat interface (Pattern B, port 7860). Three specialized agents collaborate under GroupChat with LLM-driven speaker selection, connected to the mcp-gateway's DuckDuckGo tool via AG2's native MCP SSE client. Follows compose.openai.yaml and compose.offload.yaml overrides. Updates root README classification table.

Why: Every demo in this repo uses either sequential agent delegation or a single ReAct agent. AG2's GroupChat with speaker_selection_method="auto" is the only pattern where the LLM dynamically decides which agent speaks next — a fundamentally different orchestration model not represented by any current demo. AG2 (formerly AutoGen) is the community-maintained successor to Microsoft AutoGen with a large developer community; its absence from this collection is a visible gap. The demo requires no new infrastructure: it uses the existing mcp-gateway DuckDuckGo tool, the same Docker Model Runner pattern, and standard compose overrides already used by every other Python demo.

Files added: ag2/ (10 files) + root README.md row

[VasiliyRad]

@fiam, could you please review this PR? It adds a multi-agent research team demo using AG2. AG2 (formerly AutoGen) is a community-maintained multi-agent framework.