Add real-world code examples to QUICKSTART.md

[lexasub]

Summary

Added practical real-world examples to docs/QUICKSTART.md demonstrating common AST-RAG usage patterns for three key scenarios:

1. Finding All Usages of a Function - Shows how to use ast-rag refs and ast-rag callers for refactoring
2. Understanding a New Codebase - Demonstrates using semantic search and call graphs to map architecture
3. Preparing for Refactoring - Provides step-by-step impact analysis workflow

Each example includes:

• Clear scenario description
• Practical command sequence
• Expected result explanation

These examples help developers quickly learn AST-RAG for real-world tasks.

Closes #3

Changes

• Added new "📚 Real-World Code Examples" section with three detailed examples
• Each example uses concrete function/class names (processRequest, UserService)
• Includes both CLI commands and expected outcomes
• Total addition: ~77 lines of practical guidance

Testing

• Verified git status and diff
• Successfully committed with conventional commit message
• Pushed to feature branch feat/issue-3
• PR ready for review