Add a runnable example target + end-to-end walkthrough docs

[ronaldmannak]

Motivation

The library has no runnable entry point, which makes it hard to adopt and to sanity-check the full pipeline. A small example executable doubles as living documentation.

Scope

• Add an executable target (e.g. graphrag-demo) in Package.swift under Sources/graphrag-demo/.
• Demonstrate the full flow: build a graph from sample text → ask → lightRAG().ask → detectCommunities() and print communities.
• Support both the offline path (default HashEmbedder + pattern extractor) and an Ollama path via a flag/env var.
• Add a short "Walkthrough" section to the README linking to the example.

Files (parallel-safe)

Package.swift, new Sources/graphrag-demo/, README.md. No overlap with library source changes.

Acceptance criteria

• swift run graphrag-demo runs fully offline and prints an answer, sources, and detected communities.
• README documents how to run it (offline and with Ollama).
• CI green (build includes the new target).

Context: follow-up to PRs #2 / #3.