Add github-webhook-plus plugin entry

[y9938]

Update plugin metadata.

[fguillot]

If this is a maintained fork, please publish it under a new plugin name instead of changing the ownership from kanboard/plugin-github-webhook to y9938/plugin-github-webhook.

Changing the download source effectively turns this into a supply-chain trust handoff. Using a new name makes the fork explicit and allows users to opt in intentionally.

[y9938]

@fguillot Should I send a new PR using a different branch name?

[y9938]

@fguillot I've rebranded the fork and the release zip archive now has a different name. I kept your authorship in plugins.json since this is a fork of your original repository — I made it to keep the plugin compatible with newer Kanboard versions and add some improvements. I've rebased this branch, squashing it down to a single commit that only adds the new plugin entry at the end of plugins.json.

[fguillot]

There is a validation failure: https://github.com/kanboard/website/actions/runs/24865166641/job/72964575577?pr=563

Dictionary is not alphabetically ordered:

• Violation at : 'zulip' should come after 'github-webhook-plus'

[y9938]

Now python3 scripts/validate-plugins.py passes. The branch contains one clean commit.