feat: support git hosts other than github.com, e.g. upd.dev

[xlab]

This resolves #2

Implementation

Tool has been extended to support git source hosts other than girhub.com. This may include any host, but for now I've tested with my upd.dev and also included that as an example in readme.

Note, when host is not specified, it defaults to github.com and doesn't show it in success notifications, etc. For consistency.
Also don't announce the host support in CLI docs, to not confuse agents.

Testing

% PYTHONPATH=src/agent-resources python -m agent_resources.cli.skill upd.dev/dsjacobsen/golang-pro

✅ Added skill 'golang-pro' via 🧩 agent-resources
📢 Share: uvx add-skill upd.dev/dsjacobsen/golang-pro

Note: this uses mirror on https://upd.dev/dsjacobsen/agent-resources

TODO:

I noticed this message:

💡 Create your own skill library on GitHub: uvx create-agent-resources-repo --github

I guess a PR for create-agent-resources-repo is due as well :)