Add a 'Getting started in 5 minutes' quickstart

[ahmed-sekka]

The README is comprehensive but dense. A short quickstart would lower the barrier for newcomers evaluating the project.

Acceptance criteria

• docs/quickstart.md (or top-of-README section) showing the smallest possible useful program
• One Agent, one AgentGraph, runnable with mvn exec:java
• No more than ~50 lines of Java
• Linked from the README near the top

Suggested structure

1. Add the dependency (1 snippet)
2. Define an Agent (5 lines)
3. Wire an AgentGraph (3 lines)
4. Run it (1 command, expected output shown)