Make repo clone protocol configurable #1

repo clone should follow a configurable clone protocol, similar to gh repo clone and gh config get git_protocol.

Planned behavior:

• Add a config key such as clone.protocol or git.protocol with values ssh and https.
• Use the configured protocol when the user passes OWNER/REPO.
• Respect an explicit full clone URL if the user provides one in the future.
• Keep hosted Tangled behavior aligned with the web UI, where knot1.tangled.sh repos clone through tangled.org.
• Document the behavior in README and cover URL selection with tests.