docs: add the Greptile for Open Source badge to the README #782

Add Greptile's open-source badge to the README's badge row.

Where: README.md, the centered badge block (<p align="center">, lines 7–30). The natural spot is last, after the build-status badge.

Snippet, verbatim from Greptile's OSS program:

[![Greptile: The War on Bugs](https://www.greptile.com/badge.svg)](https://www.greptile.com/?utm_source=oss_badge&utm_medium=readme&utm_campaign=greptile_for_open_source)

One catch before pasting it there: that block is raw HTML, and markdown is not rendered inside an HTML block, so the snippet above would show up as literal text. To sit inside the block, use the HTML form its neighbours use:

  <a href="https://www.greptile.com/?utm_source=oss_badge&utm_medium=readme&utm_campaign=greptile_for_open_source">
    <img alt="Greptile: The War on Bugs" src="https://www.greptile.com/badge.svg" />
  </a>

Alternatively the markdown snippet works as-is on its own line after </p>.

No other file needs a change — the badge is self-hosted by Greptile (greptile.com/badge.svg), so there is nothing to vendor and no CI step to add.