Add an option for gh stack link to create local tracking state

[onereg-felix]

Issue

gh stack link creates or updates the Stack on GitHub but intentionally creates no local gh-stack tracking state. This is useful for external stack managers, but surprising when gh stack itself is being used to link existing branches and pull requests.

After a successful link, the GitHub UI shows the Stack, while commands such as gh stack sync can still report that the current branch is not part of a stack locally.

My proposal

Add an explicit option to establish local tracking as part of the operation:

gh stack link --track-local branch-a branch-b branch-c

The option should:

• validate the proposed local branch order before changing remote state;
• refuse to overwrite conflicting local Stack metadata;
• report local and remote divergence without rebasing or force-pushing;
• leave the repository with gh stack view --json and gh stack sync working immediately.

For users who do not pass --track-local, successful output could include the command needed to import the resulting Stack:

Stack created on GitHub.
To track it locally, run `gh stack checkout <pr-number>`.