add huggingface-papers skill to agents-skills.md

[ps-abhi]

Following up on #2345 - the huggingface-papers skill exists in huggingface/skills repo but was missed when #2329 was merged. This adds it to the Available Skills table.

Link verified live.

cc @pcuenca

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Note

Low Risk
Low risk documentation-only change that adds a single new entry to the Skills table with no code or behavior impact.

Overview
Adds the missing huggingface-papers entry (with link and short description) to the Available Skills table in docs/hub/agents-skills.md.

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[pcuenca]

Nice, thank you @ps-abhi! Can we maybe place it above the paper-publisher one?

[ps-abhi]

Done, thanks for the review!

[HuggingFaceDocBuilderDev]

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