feat(save): add an atomic conflict gate for enum and relation fields

The text conflict gate (pearl--resolve-conflict) is built for mergeable text — it can open an smerge buffer, stash to the kill ring, and advance a SHA hash. None of that fits a state, priority, assignee, or label conflict, where the value is a single id or scalar that can't be combined.

pearl--resolve-atomic-conflict is the sibling for those fields: it prompts use-local / use-remote / cancel (no merge option), and delegates the actual push and the baseline advance to the caller's two callbacks, so it stays free of any field-specific mechanics. It maps to the same outcome contract as the text gate — cancel to conflict/cancelled, use-local to pushed or failed/push-failed, use-remote to resolved-remote.

It's not wired into a saver yet — the per-field savers in the next commit call it.

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