auth: tab-complete context names for `auth use`
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Add a ValidArgsFunction to entire auth use <context> so shells complete
stored context names, annotated with handle, core URL, and an (active)
marker for the current context.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: b4de22d0a34c
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