fix: use radio inputs for single-choice plan answers

[posthog]

Summary

• Plan-answer questions that are single-choice (multiSelect: false) now render a radio indicator next to each option instead of a checkbox, matching the semantics (radio = one of, checkbox = many of).
• Threaded multiSelect from ActionSelector through to OptionRow; multi-select questions still render checkboxes as before.

Test plan

• Open a task where the agent asks a single-choice plan question — confirm options render with radio buttons, selecting one deselects the other.
• Confirm a multi-select question still renders checkboxes and allows multiple selections.
• Single-question (no steps) single-select flows still work unchanged (no indicator, submits on select).

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Created with PostHog Code