issue:checkout: auto-select branch when only one exists

[Copilot]

When an issue fork has exactly one branch matching the {nid}-* pattern, the command forced an unnecessary interactive prompt or failed outright in non-interactive mode.

Changes

• Auto-selection logic — if $issueBranches contains exactly one entry and no branch argument was supplied, skip the choice question entirely and proceed directly to checkout
• User feedback — emit Auto-selected branch <name>. so the selection is visible and auditable in script output
• No behaviour change for zero branches (error), multiple branches + interactive (choice question), or multiple branches + non-interactive (error with list)

Original prompt

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This section details on the original issue you should resolve

<issue_title>If there is one branch, automatically select it in issue:checkout</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes If there is one branch, automatically select it in issue:checkout #326

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