docs: document status-dependent action attempt values

[razor-x]

Follow-up to #1022: documents the status-discriminated ActionAttempt union in the README's Action Attempts section — how error and result are typed null outside their status, with a narrow-on-status example:

if (actionAttempt.status === 'success') {
  console.log(actionAttempt.result) // The result is non-null here.
}

if (actionAttempt.status === 'error') {
  console.log(actionAttempt.error.message) // The error is non-null here.
}

Also points to the exported SucceededActionAttempt / FailedActionAttempt types. README-only change; prettier passes.

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https://claude.ai/code/session_01EdWS7o3htQ9cNxhCWL5Frp

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