feat: add problem matchers for flipt validate command

[markphelps]

Summary

Implements automatic problem matcher support for flipt validate commands, providing inline GitHub annotations on PR files when validation errors occur.

Changes

• Problem Matcher Registration: Automatically registers .github/flipt-problem-matcher.json when validate commands are detected
• JSON Format Auto-append: Automatically adds --format json to validate commands for structured error parsing
• GitHub Annotations: Creates inline annotations on PR files showing validation errors with precise line numbers
• Backward Compatibility: Preserves existing behavior when users specify their own format flags
• Comprehensive Documentation: Updated README.md and CLAUDE.md with examples and implementation details

Key Features

• Automatic activation: No user configuration required - just run flipt validate as usual
• Precise annotations: Shows exactly which file and line has validation errors
• Smart format handling: Only auto-appends --format json if user hasn't specified format flags
• Works with both v1 and v2: Compatible with all Flipt versions

Example Output

When validation errors occur, users will see annotations like:

flags.1.key: conflicting values null and string (mismatched types null and string)
File: invalid-features.yaml, Line: 8

Fixes #27