Register hidden stub commands for official GitHub extensions (gh-aw,
gh-stack) that offer to install the extension when invoked. This
replaces the error-string-matching approach from the original PR with
proper cobra commands that:
- Avoid false-positive matches on flag values or post-'--' args
- Eliminate conflicting cobra 'Did you mean?' suggestions
- Properly propagate prompt/install errors for correct exit codes
- Are hidden from help output and shell completions
- Use GroupID "extension" so checkValidExtension allows installing over them
- Are registered after extensions and aliases so both take priority
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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