fix(cli): prevent double extension in temp file for stdin markdown with --ext

[gin0606]

Fixes the issue where echo '...' | npx zx --ext='.md' produces a temp file named zx.md.md, causing Node.js to fail with ERR_UNKNOWN_FILE_EXTENSION.

After transformMarkdown() converts markdown to JavaScript, the temp file should use .mjs. However, getFilepath(dir, base) fell back to argv.ext (.md), resulting in a double extension. This fix explicitly passes EXT (.mjs) to prevent the fallback.

Before

$ printf '# Test\n\n```js\necho("hello from markdown")\n```\n' | npx zx --verbose --ext='.md'
TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".md" for /Users/gin0606/zx.md.md
    at Object.getFileProtocolModuleFormat [as file:] (node:internal/modules/esm/get_format:185:9)
    at defaultGetFormat (node:internal/modules/esm/get_format:211:36)
    at defaultLoadSync (node:internal/modules/esm/load:158:16)
    at #loadAndMaybeBlockOnLoaderThread (node:internal/modules/esm/loader:795:12)
    at #loadSync (node:internal/modules/esm/loader:815:49)
    at ModuleLoader.load (node:internal/modules/esm/loader:780:26)
    at ModuleLoader.loadAndTranslate (node:internal/modules/esm/loader:526:31)
    at #getOrCreateModuleJobAfterResolve (node:internal/modules/esm/loader:571:36)
    at afterResolve (node:internal/modules/esm/loader:624:52)
    at ModuleLoader.getOrCreateModuleJob (node:internal/modules/esm/loader:630:12) {
  code: 'ERR_UNKNOWN_FILE_EXTENSION'
}

After

$ printf '# Test\n\n```js\necho("hello from markdown")\n```\n' | node build/cli.js --verbose --ext='.md'
hello from markdown

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[antongolub]

@gin0606,

Let's try this: tempPath = tempPath || getFilepath(dir, base)

[gin0606]

Thank you for the suggestion, @antongolub!

I did consider using tempPath = tempPath || getFilepath(dir, base), but I think it may not fully address the issue in the .md case.

Here’s my understanding of the behavior when reading from stdin with --ext='.md':

1. Line 212: tempPath is set to …/zx.md via getFilepath($.cwd, 'zx', argv.ext)
2. Line 229: path.parse(tempPath) results in ext='.md' and base='zx.md'
3. Line 233: The if (ext === '.md') branch runs transformMarkdown(), converting the markdown to JavaScript
4. Line 235: The temporary file needs a .mjs extension so that Node.js can execute the transformed script

If we apply tempPath = tempPath || getFilepath(dir, base) at line 235, tempPath already has a value (…/zx.md), so that assignment would be skipped, leaving the extension as .md. In this scenario, Node.js would then fail with ERR_UNKNOWN_FILE_EXTENSION, since the file contains JavaScript but still has a .md extension.

That said, applying your suggestion at line 231 does make sense to me, as it would avoid an unnecessary reassignment when tempPath is already set. Would you be open to updating the PR to use tempPath = tempPath || getFilepath(dir, base) at line 231, while keeping the explicit EXT handling at line 235?

[gin0606]

@antongolub
Thanks for getting this merge 🙏
Looking forward to it being available in a future release.