checkout: resolve remote stack by branch name

[skarim]

• Closes Support gh stack checkout <branch> for branches that only exist on remote #427

When gh stack checkout <branch> doesn't find a locally tracked stack, it now looks for a matching branch in active stacks on remote. A match is imported through the existing PR checkout flow and switches to the requested branch.

Local matches and existing stack/PR-number precedence are unchanged. Multiple remote matches fail with guidance to use a stack or PR number. Branches that aren't part of a stack remain unsupported.

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[Copilot]

Pull request overview

Adds remote branch-name resolution to gh stack checkout, closing #427.

Changes:

• Resolves local branches first, then active remote stacks.
• Handles ambiguity and numeric branch names.
• Updates tests and documentation.

Show a summary per file

File	Description
cmd/checkout.go	Adds remote branch resolution.
cmd/checkout_test.go	Tests success, ambiguity, and API errors.
README.md	Documents remote branch checkout.
docs/src/content/docs/reference/cli.md	Updates CLI reference.
skills/gh-stack/SKILL.md	Removes the local-only limitation.
skills/gh-stack/references/commands.md	Updates checkout constraints.

Review details

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• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Balanced