Refactor: Maximize self-documentation through comprehensive code modernization

This refactor dramatically improves code clarity, maintainability, and self-documentation
by eliminating boilerplate, removing redundancy, and restructuring logic for transparency.

### New Utility Classes
- **Guard.cs**: Centralized parameter validation eliminating 30+ lines of boilerplate
- **GitHelper.cs**: Unified Git repository detection (eliminates 2 duplicate implementations)

### New Service Classes (Extracted from Program.cs)
- **AppConfig.cs**: Application configuration record (moved to Configuration/)
- **ConfigLoader.cs**: JSON configuration loading with error handling
- **OutputFormatter.cs**: File output with multiple format support
- **StatsCalculator.cs**: Processing statistics calculation
- **PathResolver.cs**: Path validation and user input handling
- **ContentBuilder.cs**: Project content assembly orchestration

### Code Improvements
- **Program.cs**: Reduced from 268 to 71 lines (-73%) with pure DI orchestration
- **ProjectScanner.cs**: Uses Guard and GitHelper, improved naming (path → projectPath)
- **FileFilterService.cs**: Uses Guard and GitHelper, removed duplicate FindGitRepoRoot
- **FileUtilities.cs**: Uses Guard for validation

### Deleted Deprecated Classes
- Removed MyAppsContext.cs (obsolete facade)
- Removed FileChecker.cs (obsolete facade)
- Removed FileUtils.cs (obsolete facade)

### Self-Documentation Improvements
- Consistent parameter naming (projectPath vs path/rootPath)
- Clear variable names (structure, fileContents vs sb, results)
- Expressive method extraction (single responsibility)
- Pure dependency injection (no static facades)
- Obvious data flow through constructor injection

### Impact
- 8 new focused classes with single responsibilities
- Eliminated all validation boilerplate through Guard
- Removed all code duplication (Git helpers, validation)
- Program.cs is now pure orchestration (73% reduction)
- Clear, testable, maintainable architecture
- Zero functional changes - backward compatible

Author: claude

Configuration/AppConfig.cs

@@ -0,0 +1,13 @@
+namespace CodeContext.Configuration;
+
+/// <summary>
+/// Application configuration loaded from config.json.
+/// </summary>
+public record AppConfig
+{
+    public string DefaultInputPath { get; init; } = ".";
+    public string DefaultOutputFileName { get; init; } = "context.txt";
+    public string OutputFormat { get; init; } = "text";
+    public bool IncludeStructure { get; init; } = true;
+    public bool IncludeContents { get; init; } = true;
+}