the generated jar should automatically include plugin.jelly and friends

When you create a plugin jar with the plugin plugin, it does not include plugin.jelly and friends by default.
Plugins in maven-plugins get that by extending the main project.xml, but for custom plugins this is annyoing. You have to add the follogin to the project.xml of the plugin:

<resources>
<resource>
<directory>src/plugin-resources</directory>
<targetPath>plugin-resources</targetPath>
</resource>
<resource>
<directory>.</directory>
<includes>
<include>plugin.jelly</include>
<include>plugin.properties</include>
<include>project.properties</include>
<include>project.xml</include>
</includes>
</resource>
</resources>

It seems to me that the plugin plugin should include those resources in the generate jar by default (at least the four files in ".", the other resources are more dependent on the structure of the custom plugin, so we can leave that for the author to configure in the pom if needed).