feat: add --json formatter

[pushpak1300]

Summary

• Adds a --json flag that outputs structured JSON instead of formatted console output
• AI agents (Claude Code, Cursor, Gemini CLI, Codex, etc.) are auto-detected via shipfastlabs/agent-detector and automatically switch to JSON output — no flags needed
• Uses a JsonOutput decorator that wraps ConsoleOutput and buffers all write()/writeln() calls, so plugins work without any modification

How it works

1. --json flag or agent auto-detection sets PEST_JSON_OUTPUT=true
2. JsonOutput replaces ConsoleOutput — plugins write normally but output is suppressed
3. The Json plugin reads test results from PHPUnit's Facade::result() and emits structured JSON
4. A shutdown handler catches early exit(1) errors and emits {"status":"error","message":"..."}

Output

Pass:

{"status":"pass"}

Fail:

{"status":"fail","failures":[{"test":"failing test","message":"Failed asserting that true is false.","location":"tests/Example.php:10","trace":"..."}]}

Token usage impact

Running Pest's test suite shows ~99% reduction in output tokens — critical for AI agents to lower costs and preserve context window space.