Support forge federation

I'd like you to consider the use-case for federated forge software.

I don't think any forges actually support issue/PR federation quite yet, but it's on the horizon for Forgejo/Codeberg and I'm wondering what that use-case looks like for this project. If the "platform" key is just matched as a string, then you could just use the full domain name of the forge for the platform string, and the script would know that platform.tld:user is the federated user user@platform.tld, job done.

I don't quibble with your choice of representation in the .td file, I just want to suggest that the platform key could be standardized as a domain name, instead of a string like github which implies the github.com domain and API.

I think this project is a great idea and I hope to see it succeed 😄

Start by locating how the platform key is represented in .td files and how the script matches platform values. Compare the proposed domain-based representation with the current string form and identify compatibility concerns. Done means the federation use case has a clear implementation or documentation decision, with the affected behavior covered.