Pass explicit paths to PHPCS even when project has a custom config

[mhdarffn]

Since 3.4.0, PHP_CodeSniffer ignores any paths the user passes to duster lint / duster fix whenever a custom phpcs config exists at the project root (.phpcs.xml, phpcs.xml, .phpcs.xml.dist, phpcs.xml.dist). The dirty list resolved by App\Project::paths() is correctly stored in DusterConfig, but PhpCodeSniffer::lint() and fix() short-circuit on hasCustomConfig() and call process() with an empty array, so PHPCS falls back to the <file> directives in the ruleset and scans the whole project.

This silently breaks --dirty (and --diff) for any project that has a custom phpcs config — every commit re-lints the entire codebase through pre-commit hooks instead of just the changed files.

Refactors the path resolution into a single resolvePaths() helper with three outcomes:

• null — skip PHPCS entirely (e.g. only Blade files
  were passed explicitly; preserves PR Manually exclude Blade files from calls to PHPCS #199).
• [] — defer to the ruleset's <file> directives
  (no explicit paths, custom config present;
  preserves existing 3.4.x behaviour).
• array<string> — pass paths through so PHPCS uses them
  instead of the ruleset's <file> list.
  Restores --dirty / --diff / explicit
  path argument behaviour for projects with
  a custom phpcs config.

Adds a regression test (it lints only the explicit paths when project has a custom phpcs config) and pins the fall-back-to-ruleset behaviour so it does not regress in future.

Fixes #206