Require project PHPStan config and remove hidden wrapper fallback behavior

[UltraBob]

Closes #15

Summary

• Require project PHPStan config (phpstan.neon*) unless --configuration is provided.
• Remove hidden fallback to add-on assets/phpstan.neon.
• Remove runtime scope/exclude and default-path injection from wrapper.
• Remove runtime baseline include temp-config injection.
• Keep post---generate-baseline config update behavior.

UX change

When project config is missing, wrapper now fails fast with a clear message explaining how to add phpstan.neon or pass --configuration.

Docs

• README updated to describe PHPStan config requirement.
• docs/tooling/config-sources.md updated to remove fallback/injection behavior.

Test

• Added one focused test: phpstan fails with helpful message when project config is missing.