Add Devin CLI plugin manifest (#318)

Adds `.devin-plugin/plugin.json` so the plugin can be installed with
`devin plugins install DietrichGebert/ponytail`. The skills/ directory
is already at the repo root, which is exactly where Devin expects it.

Registers the new manifest in `scripts/check-versions.js` so it stays
in sync with every other host manifest on release.

Updates README: install/uninstall instructions, badge (14 → 15 agents),
and Devin added to the skill-capable hosts list in Commands.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>