Replace tomasvotruba/unused-public with shipmonk/dead-code-detector

[TomasVotruba]

Swaps the dev dependency tomasvotruba/unused-public for shipmonk/dead-code-detector.

• removes tomasvotruba/unused-public, requires shipmonk/dead-code-detector (auto-registers via phpstan/extension-installer)
• drops the unused-public specific public.method.unused ignore from phpstan.neon
• adds phpstan-baseline.neon for the new dead-code findings, which are all DI container and console entrypoints (command constructors and run() methods, plus DI service constructors) that the detector cannot trace statically

CI green locally: PHPStan, ECS, Rector, PHPUnit, composer validate, commented-code and class-leak checks.