fix(Runner): check mkdir() return value to prevent silent failures

[s2x]

Closes #151

Checks the return value of mkdir() in Runner::run() and throws a clear \RuntimeException on failure. The double is_dir() check handles the race condition where another process may create the directory between the initial check and the mkdir() call.

[s2x]

Code Review

Warnings

1. Missing test for mkdir() failure path
   The project uses structural tests in RunnerTest.php to verify error-handling patterns (e.g., testRunnerUsesCorrectForkErrorHandling). The new mkdir() failure branch should have a matching assertion to prevent regression and maintain consistency with the existing test strategy.

2. Missing CHANGELOG entry
   The [Unreleased] section is currently empty. Per CONTRIBUTING.md, please add a Fixed entry under [Unreleased] following the Keep a Changelog format.

Nits

1. Inconsistent sprintf namespacing
   The new code uses sprintf() without the leading backslash, while existing line 71 uses \sprintf(). Please use \sprintf() for consistency with the rest of the file.

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PR Health: CI passing | Not draft | Awaiting approval