feat: register the snyk agent (AXI) extension

[mattdolan-snyk]

Wire github.com/snyk-playground/cli-extension-axi into the CLI:

• initExtensions registers agent.Init, adding the snyk agent command surface (agent-ergonomic, TOON output): agent, agent test [scanners...], agent context, agent setup hooks.
• createCommandsForWorkflows sets a help func on the agent command (inherited by the subtree) that delegates to agent.RenderHelp with the engine, so the extension can render per-command help by introspecting each scanner's underlying workflow flags — without hardcoding other extensions' flags.

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• Updates relevant GitBook documentation (PR link: ___)
• Includes product update to be announced in the next stable release notes

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