orchestrator: merge preview trusts git's exit code and NUL-separated paths

Some conflicts (directory renames) name no file, so clean comes from the
exit code; -z avoids quotePath escapes; duplicate paths are dropped. The
CLI refuses --keep-branch with --discard and says which flag applies.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
da9efe10d1Lucenx9 committed on 10/4/2026, 2:20:58 PMparent9cfc3c0

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