docs: fill docstring gaps identified in Phase 3 audit

- DeleteOp, MoveOp, ReplaceOp: document every field (consistent with InsertOp) - MergeResult.is_clean: add property docstring - write_merge_state(): add Args block for all 6 keyword-only params - apply_resolution(): document destructive muse-work/ side-effect in docstring - diff_snapshots(): add Args/Returns block - apply_merge(): add full Args/Returns with merge semantics prose - MusicPlugin.merge(): add Args/Returns block; clarify left/right vs ours/theirs - _diff_modified_file(): add full Args/Returns with fallback condition - _read_branch, _read_repo_id, _restore_from_manifest: add one-liner docstrings