Add unseal example and docs for already-initialized vaults

[Copilot]

Users had no guidance on how to unseal a vault that was already initialized — the existing example/init.js only covers the combined init+unseal flow.

• example/unseal.js: Standalone example reading UNSEAL_KEY from env, consistent with other examples
• README.md: New section covering single-key and multi-key (threshold > 1) unseal scenarios

// single key
vault.unseal({ key: 'my-unseal-key' })
  .then(console.log)
  .catch(console.error);

// threshold > 1: call unseal repeatedly, checking result.sealed and result.progress
vault.unseal({ key: 'first-key' })
  .then((result) => {
    console.log('Progress:', result.progress + '/' + result.t);
    return vault.unseal({ key: 'second-key' });
  });

Original prompt

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This section details on the original issue you should resolve

<issue_title>How to unseal a vault that is already initialized?</issue_title>
<issue_description>Basic question. I don't see any examples on handling this scenario.
Thanks</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes How to unseal a vault that is already initialized? #211

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