CLI stack sync should detect an existing remote Stack

[onereg-felix]

Issue

Related discussions: #220, #158, and #139.

A Stack can exist on GitHub while its branches are not tracked locally by gh stack. In that state, the GitHub UI correctly shows the Stack, but running gh stack sync from one of its branches reports that the branch is not part of a stack. This makes the local CLI and GitHub UI appear to disagree, and the recovery path is not discoverable from sync.

GitHub Stack membership and local gh-stack tracking are separate states, but that distinction is not visible to users. Detecting the remote Stack would make sync resilient when a Stack was created from another machine, through the web UI, or using gh stack link.

Rep steps

1. Create or link a Stack on GitHub, for example with gh stack link or the web UI.

2. Check out one of the Stack's branches in a local clone that does not have the corresponding gh-stack metadata.

3. Run:

   gh stack sync

4. The command reports:

   current branch "feature-part-2" is not part of a stack
   

The branch's pull request is nevertheless part of a Stack on GitHub.

Expected behaviour

gh stack sync should check whether the current branch has an open pull request that belongs to a remote Stack before concluding that it is unstacked.

When exactly one Stack is found, the CLI could:

• offer to import and track that Stack locally in an interactive terminal;
• provide a non-interactive flag such as --import-remote; or
• print the exact gh stack checkout <pr-number> command required to establish local tracking.

The command should perform no rebase or push until the remote Stack has been imported successfully and any local divergence has been reported.