Add Repository::runProcess() to expose stderr on successful commands

[lyrixx]

Summary

Repository::run() only ever returns stdout, even when the process succeeds. Some git commands (push, in particular) write their meaningful output to stderr even on success, so callers had no way to read it.

Fixes #205.

Changes

• Extracted the process-running logic (logging, debug output, error handling) out of run() into a new public method runProcess(), which returns the full, already-run Symfony\Component\Process\Process instead of just stdout.
• run() now delegates to runProcess() and keeps its exact original behavior (stdout-only, null on failure, throws in debug mode) — fully backward compatible.
• Callers who need stderr (e.g. for push) can now do:

  $process = $repository->runProcess('push', ['origin', 'main']);
  $process->getErrorOutput();

Test plan

• Added tests covering runProcess() exposing stderr on success and returning a failed (non-throwing) process when debug is off
• Full test suite passes (vendor/bin/phpunit)
• PHPStan level 9 clean
• php-cs-fixer clean