Switch agent output from JSON to TOON for ~19% token reduction

[pushpak1300]

This pull request switches the AI agent output format from JSON to TOON (Token-Oriented Object Notation) — a compact, machine-parseable format optimized for LLM contexts.

Follows up on #4938.

Why:

TOON saves ~15% tokens compared to JSON while keeping the same data structure fully machine-parseable. It achieves this by declaring field names once as a header (messages[9]{message,line,ignorable,identifier}:) and encoding each error as a single comma-separated line — no repeated keys, braces, or brackets per entry.

Token comparison (9 real errors, measured with o200k_base tokenizer):

Format	Characters	Tokens	vs JSON
table	2,667	379	-4%
json	1,705	394	baseline
toon	1,377	334	-15%

How detection works:

1. When Claude Code runs PHPStan, it sets the CLAUDECODE / CLAUDE_CODE environment variable
2. When Cursor runs PHPStan, it sets CURSOR_TRACE_ID / CURSOR_AGENT
3. Other agents set their own env vars (GEMINI_CLI, CODEX_SANDBOX, AI_AGENT, AUGMENT_AGENT, OPENCODE, REPL_ID, etc.)
4. PHPStan detects these via shipfastlabs/agent-detector and automatically switches to TOON format + suppresses progress bar animations — no flags needed

What changed:

1. Added helgesverre/toon dependency for TOON encoding
2. Created ToonErrorFormatter — builds the same error data structure as JsonErrorFormatter but encodes as TOON
3. Agent detection now switches to toon format instead of json
4. TOON format is also available directly via --error-format=toon

Output:

totals:
  errors: 0
  file_errors: 1
files:
  "/path/to/file.php":
    errors: 1
    messages[1]{message,line,ignorable,identifier}:
      "Error message",10,true,error.id
errors[0]:

Explicit --error-format=table or config errorFormat: table always takes priority over agent detection.

related laravel/pint#415

[phpstan-bot]

You've opened the pull request against the latest branch 2.2.x. PHPStan 2.2 is not going to be released for months. If your code is relevant on 2.1.x and you want it to be released sooner, please rebase your pull request and change its target to 2.1.x.

[VincentLanglet]

I wonder what's your policy about added library in PHPStan @ondrejmirtes ?
Adding library with low number of star and/or low number of contributors/maintainers is always a risk (especially for security reason).
I always have this story in mind

[ondrejmirtes]

Not worth it here...

[ondrejmirtes]

I said you should try raw format instead 😊

[VincentLanglet]

It could still be exposed as a standalone extension pushpak1300/phpstan-toon-formatter maybe ?

I'm not sure about it but I think PHPStan expose enough extension point to add custom formatter, no ?