refactor: Modularize ContextGenerator::generate for Better Maintainability

[BETAER-08]

Problem

The ContextGenerator::generate function in src/core/generator.rs is currently a massive, monolithic block of code. It handles multiple responsibilities: resolving focus targets, fallback vector searching, expanding the call graph, and formatting the Markdown string. This violates the Single Responsibility Principle and makes the core logic hard to maintain.

Proposed Solution

• Extract specific responsibilities into smaller, private helper functions within ContextGenerator:
  • resolve_focus_targets(query) -> Vec<String>
  • expand_graph_depth(initial_targets, depth) -> Vec<String>
  • format_markdown_report(files, edges) -> String
• Clean up the main generate function to act simply as an orchestrator.

Definition of Done

• generate function is broken down into smaller, logical sub-functions.
• Core functionality remains identical (no regression in Markdown output).
• Code readability and maintainability are vastly improved.