Extend the port-labs/port-github-action to support invoking AI Agents - via the Invoke a specific agent API endpoint.

Our use-case is to use GitHub Actions to drive an automated (schedule-driven) refresh of metrics summaries. I.e. we have a markdown widget on a dashboard, and will use an AI Agent to summarise metrics values, then post this summary to the markdown widget on a regular cadence.

Note: it's a scheduled refresh hence not driving this from within Port automation.