checkout: detect remote stack from current branch

[skarim]

• Closes Support gh stack checkout (no args) to auto-detect and checkout the stack for the current branch #426

When gh stack checkout runs without a target, it now checks whether the current branch is not part of a local stack but belongs to an active stack on GitHub. If found, it offers to check out that stack locally.

The lookup remains best-effort: network failures, no or multiple matches, and declined confirmation fall through to the existing picker. Fully merged stacks are ignored, Ctrl+C still aborts, and non-interactive behavior is unchanged.

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[Copilot]

Pull request overview

Adds best-effort remote stack detection to interactive checkout.

Changes:

• Detects a unique remote stack containing the current branch.
• Adds paginated stack retrieval and tests.
• Updates checkout documentation.

Show a summary per file

File	Description
README.md	Documents auto-detection.
cmd/checkout.go	Implements detection and confirmation.
cmd/checkout_picker_test.go	Tests detection behavior.
internal/github/github.go	Paginates stack listing.
internal/github/github_test.go	Tests pagination.
docs/src/content/docs/reference/cli.md	Updates CLI reference.

Review details

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• Files reviewed: 6/6 changed files
• Comments generated: 3
• Review effort level: Balanced