Keep the leading slash in pretty output path for index files

[lancechentw]

Found that $this->outputPathResolver->link() would generate path without a leading slash for index files, e.g. index.blade.php, index.html, index.md, when using pretty urls (which is the default!)

This causes problem when we have source/blog/index.blade.php instead of source/blog.blade.php. link() would return blog for the former file, and return /blog for the latter file.

For example, GenerateSitemap.php from the blog template would generate the following if having source/blog/index.blade.php

   <url>
-   <loc>https://jigsaw-blog-template.tighten.co/blog</loc>
+   <loc>https://jigsaw-blog-template.tighten.coblog</loc>
    <lastmod>2025-11-27T07:21:16+00:00</lastmod>
    <changefreq>daily</changefreq>
   </url>

I have traced back to the commit https://github.com/tighten/jigsaw/commit/7aa603ab2d5177eaf13a2c4e46b6512789d925f4#diff-76e91e6d5ebc8f831ce78c19c83441aa3b719af6a3d4098823f9195a639ec285R11 that introduced the change, and I think it was merely a bug, ltrim() should be applied right before $this->trimPath().