Improve repo location in Deep Link service for PRs

[axosoft-ramint]

The "add repo" case in the deep link service, which the Launchpad "Open in Worktree" command uses, doesn't leverage everything we know about the PR in the Launchpad. It should be able to leverage functions like getOrOpenPullRequestRepository from pullRequest.ts (using whatever we can pull through over the link like head and base repo URLs).

Right now the "add repo" case only cares about the remote url, which is not as in-depth. We should expand it to do more to get to the opened repo when the target is a PR.