chore: replace hack with tools and remove obsolete artifacts

- Rename hack/ to tools/ for formal naming
- Update plugin development and deployment script references
- Remove obsolete DESIGN/MILESTONE/PLAN docs now superseded by docs/
- Remove abandoned internal/onebot implementation
- Ignore stray local qq binary artifact

tools/plugin-dev/templates/plugin.json.tmpl

@@ -0,0 +1,10 @@
+{
+  "name": "{{.Name}}",
+  "version": "{{.Version}}",
+  "description": "{{.Description}}",
+  "author": "{{.Author}}",
+  "license": "MIT",
+  "entry": "plugin.so",
+  "min_version": "1.0.0",
+  "tags": ["{{.Name}}"]
+}