feat(tags): Implement tagged task lists migration system (Part 1/2)

This commit introduces the foundational infrastructure for tagged task lists,
enabling multi-context task management without remote storage to prevent merge conflicts.

CORE ARCHITECTURE:
• Silent migration system transforms tasks.json from old format { "tasks": [...] } 
  to new tagged format { "master": { "tasks": [...] } }
• Tag resolution layer provides complete backward compatibility - existing code continues to work
• Automatic configuration and state management for seamless user experience

SILENT MIGRATION SYSTEM:
• Automatic detection and migration of legacy tasks.json format
• Complete project migration: tasks.json + config.json + state.json
• Transparent tag resolution returns old format to maintain compatibility
• Zero breaking changes - all existing functionality preserved

CONFIGURATION MANAGEMENT:
• Added global.defaultTag setting (defaults to 'master')
• New tags section with gitIntegration placeholders for future features
• Automatic config.json migration during first run
• Proper state.json creation with migration tracking

USER EXPERIENCE:
• Clean, one-time FYI notice after migration (no emojis, professional styling)
• Notice appears after 'Suggested Next Steps' and is tracked in state.json
• Silent operation - users unaware migration occurred unless explicitly shown

TECHNICAL IMPLEMENTATION:
• Enhanced readJSON() with automatic migration detection and processing
• New utility functions: getCurrentTag(), resolveTag(), getTasksForTag(), setTasksForTag()
• Complete migration orchestration via performCompleteTagMigration()
• Robust error handling and fallback mechanisms

BACKWARD COMPATIBILITY:
• 100% backward compatibility maintained
• Existing CLI commands and MCP tools continue to work unchanged
• Legacy tasks.json format automatically upgraded on first read
• All existing workflows preserved

TESTING VERIFIED:
• Complete migration from legacy state works correctly
• Config.json properly updated with tagged system settings
• State.json created with correct initial values
• Migration notice system functions as designed
• All existing functionality continues to work normally

Part 2 will implement tag management commands (add-tag, use-tag, list-tags) 
and MCP tool updates for full tagged task system functionality.

Related: Task 103 - Implement Tagged Task Lists System for Multi-Context Task Management