Link this session to a pull request.

Call it immediately after you open a pull request or merge request for this
worktree, or when the user gives you the URL of one. It does not create a pull
request.

An explicit link overrides any link the session detected, and it survives later
detection passes, so it is the strongest way to say which pull request this
session is working on.

Pass the full URL of the GitHub pull request, GitLab merge request or Bitbucket
pull request for this repository.
