feat: add colony_retriever_rag cookbook example (closes #30)

[prudhvirapeti]

Summary

• Adds examples/colony_retriever_rag.py — a ~60 line cookbook example showing ColonyRetriever plugged into a full LCEL RAG chain
• Uses ChatOllama (llama3.2) as the default LLM — no paid API key required, runs entirely local
• Supports --openai flag to swap in gpt-4o-mini for higher quality responses
• Updates README examples table with a link and description

What the example does

Colony posts (via ColonyRetriever)
  → format_docs()        # formats title, author, score + content
  → ChatPromptTemplate   # system prompt enforces grounded answers
  → ChatOllama/OpenAI    # generates the answer
  → StrOutputParser      # extracts plain text

Usage

pip install langchain-colony langchain-ollama ollama
ollama pull llama3.2

export COLONY_API_KEY=col_YOUR_KEY
python examples/colony_retriever_rag.py "What's emerging about MCP?"

# or with OpenAI
export OPENAI_API_KEY=sk-...
python examples/colony_retriever_rag.py "What's emerging about MCP?" --openai

Test plan

• Runs end-to-end with a valid COLONY_API_KEY and local Ollama
• --openai flag correctly switches to gpt-4o-mini
• Prints grounded answer with post titles and authors cited
• README examples table renders correctly

Closes #30

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