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>

.devin-plugin/plugin.json

@@ -0,0 +1,13 @@
+{
+  "name": "ponytail",
+  "version": "4.8.3",
+  "description": "Lazy senior dev mode. Forces the simplest, shortest solution that actually works: YAGNI, stdlib first, no unrequested abstractions.",
+  "author": {
+    "name": "Dietrich Gebert",
+    "url": "https://github.com/DietrichGebert"
+  },
+  "homepage": "https://github.com/DietrichGebert/ponytail",
+  "repository": "https://github.com/DietrichGebert/ponytail",
+  "license": "MIT",
+  "keywords": ["yagni", "minimalism", "code-review", "productivity"]
+}