[reporting] Report unused skips in JSON output too

[TomasVotruba]

Why

->reportUnusedSkips() reports skips that never matched, so you can clean up dead ->withSkip() entries. It writes via SymfonyStyle, but when --output-format=json is used, ProcessCommand forces VERBOSITY_QUIET and the warning is silenced entirely. Editor/CI integrations typically run --dry-run --output-format=json, so unused skips never showed up there.

What

Surface unused skips in the JSON payload too. The computation is extracted into a shared UnusedSkipResolver service, so console and JSON stay in sync.

Example

Config:

return RectorConfig::configure()
    ->withRules([SimplifyUselessVariableRector::class])
    ->reportUnusedSkips()
    ->withSkip([
        SimplifyUselessVariableRector::class => [
            __DIR__ . '/src/Used.php',          // matched -> not reported
            __DIR__ . '/src/NeverMatches.php',  // never matched -> reported
        ],
    ]);

Console output (already worked):

 [WARNING] This skip is unused, it never matched any element. You can remove it
           from "->withSkip()"

 * Rector\CodeQuality\Rector\FunctionLike\SimplifyUselessVariableRector => /src/NeverMatches.php

JSON output before -- unused skips silently missing:

{
    "totals": { "changed_files": 1, "errors": 0 },
    "file_diffs": [ ... ],
    "changed_files": [ "src/SomeClass.php" ]
}

JSON output after -- new unused_skips key:

{
    "totals": { "changed_files": 1, "errors": 0 },
    "file_diffs": [ ... ],
    "changed_files": [ "src/SomeClass.php" ],
    "unused_skips": [
        "Rector\\CodeQuality\\Rector\\FunctionLike\\SimplifyUselessVariableRector => /src/NeverMatches.php"
    ]
}

The key is added only when reportUnusedSkips() is enabled and something is actually unused, so the JSON schema is otherwise untouched.

Changes

• src/Reporting/UnusedSkipResolver.php (new) -- single source of truth. Returns "rule => path" for rule-scoped skips and a plain path for global ones; honors the reportUnusedSkips() guard; keeps the existing rules (per-path granularity, rule-scoped masks kept, global masks excluded).
• src/Reporting/MissConfigurationReporter.php -- delegates to the resolver; console output unchanged.
• src/ChangesReporting/Output/JsonOutputFormatter.php -- injects the resolver, passes results to the factory.
• src/ChangesReporting/Output/Factory/JsonOutputFactory.php -- optional $unusedSkips param, adds the unused_skips key when non-empty.

Tests

• New UnusedSkipResolverTest (unit): rule-scoped unused skip reported as rule => path; matched / global-mask / skip-everywhere excluded; disabled returns empty.
• MissConfigurationReporterTest updated to the new constructor.