feat(install): add shared skill install workflow and cobra adapter

[samzong]

Summary

Adds a shared bundled skill install workflow across TypeScript, Go, and Rust, including standard flag parsing, scope and agent selection, confirmation handling, structured exit helpers, embedded bundle support, and a thin optional Go Cobra adapter.

Scope Check

• Stays within the v0.1 bundled-skill installer boundary.
• Does not add marketplace, registry, remote install, script execution, GUI, MCP server, or agent runtime behavior.
• Updates golden cases for observable installer behavior changes.
• Regenerates host constants after spec/hosts.json changes.

Verification

• make check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Return an error when the workflow is canceled

When an interactive user cancels at the confirmation prompt or submits an empty multi-select, ClassifyInstallWorkflowExit marks the workflow as OK: false with code canceled, but this helper swallows that state and returns nil. The new Cobra adapter returns InstallWorkflowError(report) directly, so a canceled install exits successfully instead of signaling cancellation to the CLI caller; return the canceled message as an error (or have the adapter use the classifier) rather than treating it as success.

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