Pasting a link whose text/html label is a shortened form of its URL plants a `[` inside the URL

### What happens

When a clipboard carries a URL as `text/plain` and an anchor labelled with a *shortened rendering* of that URL as `text/html`, pasting into a GitHub comment box inserts the Markdown link syntax **inside** the URL:

```
text/plain: https://github.com/owner/repo/blob/main/a.js#L7
text/html: repo/blob/main/a.js#L7

pasted: https://github.com/owner/[repo/blob/main/a.js#L7](https://github.com/owner/repo/blob/main/a.js#L7)
expected: [repo/blob/main/a.js#L7](https://github.com/owner/repo/blob/main/a.js#L7)
```

It renders as two links, one of them junk (`https://github.com/owner/` on its own), and the comment has to be repaired by hand.

### Reproduction

Run this in the console of any page with a GitHub comment box, press ⌘C/Ctrl+C, then paste into the box:

```js
document.addEventListener('copy', event => {
const url = 'https://github.com/owner/repo/blob/main/a.js#L7'
event.clipboardData.setData('text/plain', url)
event.clipboardData.setData('text/html', `repo/blob/main/a.js#L7`)
event.preventDefault()
}, {once: true})
```

### Why it happens

`convertToMarkdown` starts from the plaintext flavour and splices `[label](href)` over the label at the offset where `indexOf` finds it:

https://github.com/github/paste-markdown/blob/main/src/paste-markdown-html.ts#L83-L90

That is correct for prose containing a link, where the plaintext is a flattening of the HTML. Here the plaintext *is* the URL and the label is a substring of it, so the label is found at offset 25 rather than 0 and the splice covers only the tail.

The same corruption happens when prose wraps the URL in punctuation — plaintext `(https://github.com/owner/repo/blob/main/a.js#L7)` yields `(https://github.com/owner/[repo/…#L7](…))`.

### Who hits it

Any tool that writes a link to the clipboard as "URL as plaintext, shortened label as HTML" — macOS automation scripts (Hammerspoon/Alfred/Keyboard Maestro), clipboard managers, and "copy link" affordances that shorten the visible text. Labels containing a character the URL does not (`repo#1234`) are unaffected, because `indexOf` misses and the paste is left alone; every label that is a substring of its own URL is affected.

### Proposed fix

#117 — replaces the whole URL when the label is a shortened rendering of it, declines rather than corrupting when the label sits inside a URL that is not its own href, and leaves every other case on the existing path. Four regression tests, all failing before the change.

Start in src/paste-markdown-html.ts around lines 83-90, where convertToMarkdown locates the HTML label in the plaintext. Reproduce the clipboard case from the issue, then add the four regression tests described in the proposed fix and run the existing test suite. Done means shortened URL labels are replaced correctly without corrupting URLs, while other cases retain their current behavior.

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