[ax] Add --output-format=json to validate-config

[TomasVotruba]

Stacked on #8450 (base branch tv-validate-config); will retarget to main once #8450 merges.

Why

validate-config (#8450) prints human warnings and sets an exit code. Agents and CI want a machine-readable result they can parse, not scrape from console text.

What

validate-config --output-format=json emits a compact machine gate to stdout and suppresses the human warnings:

{
    "valid": true,
    "issue_count": 0
}

• exit 0 when valid is true, 1 otherwise - same gate as console mode.
• default stays console.
• warnings are silenced in json mode (verbosity QUIET), so stdout is clean JSON only, matching how process --output-format=json behaves.

Scope note

This v1 payload is the pass/fail gate (valid + issue_count). Per-finding detail (which rule/args are deprecated) would require the hygiene reporters to return structured findings instead of just counts - a larger change also touching ProcessCommand - so it is left as a follow-up.

Tests

• testJsonOutputOnCleanConfig / testJsonOutputOnDeprecatedRegisteredRule assert both the payload and the exit code (output captured via ob_*).
• composer check-cs, composer phpstan clean; verified on the CLI.