Reformat repository structure

Reformat the plugins repository based on the following structure

```
fluxnova-plugins/ # engine-specific plugins
├─ LICENSE
├─ NOTICE
├─ README.md
├─ pom.xml # parent POM
├─ .github/
│ └─ workflows/
│ ├─ ci.yml # mvn verify
│ └─ release.yml # Maven Central publish
├─ tooling/
│ ├─ parent-pom/ # shared config
│ └─ archetype/ # plugin scaffolding
└─ plugins/
├─ mcp-server-plugin/
│ ├─ pom.xml
│ ├─ README.md
│ └─ src/
│ ├─ main/java/
│ └─ test/java/
└─ mcp-start-event/
├─ pom.xml
└─ src/
```

Based on discussion here:
https://github.com/finos/fluxnova-bpm-platform/discussions/139