update add cmd to adopt existing branches

[skarim]

Allow gh stack add to adopt existing branches

Previously, gh stack add rejected any branch name that already existed
in git with a blanket "branch already exists" error. This was overly
restrictive — users who create branches ahead of time (e.g. from the
GitHub UI or via git branch) had no way to incorporate them into a
stack without deleting and recreating them.

Now, if the specified branch exists in git but is not part of any
existing stack, add adopts it: it skips branch creation, checks out
the existing branch, and appends it to the stack metadata. This mirrors
the adopt-or-create pattern already used by gh stack init.

Branches that belong to another stack are still rejected by the existing
ValidateNoDuplicateBranch guard, so there is no risk of cross-stack
conflicts.

Behavioral summary:

• Existing branch, not in any stack → adopted (checkout only, no create)
• Existing branch, already in a stack → error (unchanged)
• Non-existent branch → created (unchanged)
• Staging/commit flags (-A, -u, -m) work with adopted branches

Tests added:

• TestAdd_AdoptsExistingBranch
• TestAdd_RejectsExistingBranchInStack
• TestAdd_AdoptsExistingBranchWithCommit

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