feat: refactor filter

[timmilesdw]

Overview

Major refactoring of the shell-operator filter system: replaced complex interface-based architecture with simplified functional approach, consolidated jq filtering logic, and added comprehensive test coverage.

What this PR does / why we need it

Key Changes:

• 🔄 Architecture Overhaul: Replaced Filter interface with functional Run() API
• 📁 Consolidation: Removed pkg/filter/jq/ subdirectory, merged logic into main package
• 🎯 API Simplification: Single function with clear precedence (custom filter > JQ expression > no filter)
• ✅ Enhanced Testing: Added 614 lines of comprehensive unit tests
• 🔧 Integration: Updated 15+ files across codebase for new API

Benefits:

• Simplified architecture and maintenance
• Clear filter precedence rules
• Consolidated codebase
• 100% backward compatibility maintained
• All tests pass with no performance regression

Special notes for your reviewer

Internal refactoring only - no breaking changes for external consumers. Major code simplification while preserving all existing functionality.