responder: agent for @qu0b-reviewer question mentions

[qu0b]

Adds agents/responder/ — consumed by the events-ingress container's new question mode (mention @qu0b-reviewer in a PR comment to ask a question).

Same model config (starflinger-anthropic, thinking=high) and reviewer toolset as the review agent; the prompt is tuned for answering one question, grounded in the checked-out PR head, with the output posted verbatim as a reply comment.

Validation gate covers it automatically (agents/*/agent.json glob).

[redpandabot]

🤖 qu0b-reviewer

Summary

Adds a second agent, responder — agent.json is a near-verbatim copy of the reviewer's config (same model/provider/toolset, validated), with a new, focused prompt for answering @qu0b-reviewer mentions in PR comments. Config is valid, and scripts/validate.sh (the CI gate) passes cleanly for both agents; nothing functionally wrong in the new code.

Suggestions

• README.md:7-9 — the "Currently shipped" table lists only reviewer, but since merge-to-main is the deploy (per the README's own CI/deploy notes), the newly-added responder agent ships without a matching table entry; add a row so the doc stays in sync with the shipped agents.

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_{Reviewed @ 74426c61
"Even a stopped clock is right twice a day."}