fix: add clipboard fallback for non-secure contexts (HTTP)

[ptlzc]

Problem

The Clipboard API (navigator.clipboard.writeText) is only available in secure contexts (HTTPS or localhost). When agentation runs over plain HTTP — e.g. a Vite dev server inside a Docker container accessed by IP — navigator.clipboard is undefined and the copy button silently fails. The UI shows the "copied" checkmark, but nothing actually reaches the clipboard.

The existing catch block swallows the error without any fallback:

if (copyToClipboard) {
  try {
    await navigator.clipboard.writeText(output);
  } catch {
    // Clipboard may fail (permissions, not HTTPS, etc.) - continue anyway
  }
}

Fix

Add a copyTextToClipboard utility that:

1. Tries navigator.clipboard.writeText() first (preferred, secure contexts)
2. Falls back to a temporary <textarea> + document.execCommand('copy') when the Clipboard API is unavailable

This ensures copy works in all contexts — HTTPS, localhost, and plain HTTP.

Testing

• All 83 existing tests pass
• Build succeeds (tsup CJS + ESM + types)
• No new dependencies

[vercel]

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