Craft post-publish merge is non-blocking housekeeping

After all publish targets complete, handleReleaseBranch() in src/commands/publish.ts merges the release branch back into the default branch and deletes it. This is a housekeeping step — failures are caught, reported to Sentry via captureException, and logged as warnings without failing the command. The merge uses --no-ff with the default (ort) strategy first, then retries with -s resolve if conflicts occur (handles criss-cross ambiguities in files like CHANGELOG.md). An isAuthError() helper distinguishes authentication failures (expired tokens) from merge conflicts to provide targeted diagnostics.