Add gemini-agent Prow plugin

[ameukam]

Implements a Prow plugin triggered by /gemini-agent comments on GitHub issues and PRs. The plugin collects GitHub context (issue metadata, PR diffs, recent comments) and sends it to Gemini on Vertex AI for analysis.

Assisted by Opus 4.6

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[ameukam]

/assign @petr-muller @krzyzacy

[petr-muller]

TBH I'm not a fan of these integrations being internal plugins (how many /X-agent implementations would we maintain inside hook?), I think they have a potential to grow in functionality and in how they are coupled with the given AI service to be standalone external plugins.

[ameukam]

TBH I'm not a fan of these integrations being internal plugins (how many /X-agent implementations would we maintain inside hook?), I think they have a potential to grow in functionality and in how they are coupled with the given AI service to be standalone external plugins.

I don't think we have clear policy on what's eligible for internal plugins. it's not only the AI service but everything that's interact with a third-party service.
I'll defer do your decision but IMHO we should provide more clarity on internal.

[midnightconman]

Internal / external plugins should have better docs, but the gist of it is that external plugins (like cherrypicker) aren't compiled into hook and as such are easier to manage.

@petr-muller if this was converted to an external plugin, could it still be staged here in the prow repo as an example?

• https://docs.prow.k8s.io/docs/components/external-plugins/cherrypicker/
• https://github.com/kubernetes-sigs/prow/tree/main/cmd/external-plugins

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This pull-request has been approved by: ameukam
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[ameukam]

@petr-muller @midnightconman rewritten as an external plugin. PTAL!. 🙏🏾