feat(skills): add arXiv literature search skill

[pikaqiu2333]

• A human has tested these changes.

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Why

OpenHands has general research and citation skills, but not a focused academic literature-search skill for arXiv-heavy workflows. This adds operational guidance for API-first literature search, arXiv-safe throttling, OpenAlex/Semantic Scholar metadata enrichment, caching, deduplication, and browser fallback boundaries.

Summary

• Add an arxiv-literature-search skill covering arXiv, OpenAlex, Semantic Scholar, provider rate limiting, caching, deduplication, and browser fallback rules.
• Register the skill in marketplaces/openhands-extensions.json with plugin metadata and vendor symlinks.
• Regenerate the README catalog and skills/index.js.

Issue Number

N/A

How to Test

• npm run build:skills
• .venv\\Scripts\\python -X utf8 scripts\\sync_extensions.py catalog coverage --check
• .venv\\Scripts\\python -X utf8 -m pytest tests\\test_skills_have_readme.py
• .venv\\Scripts\\python -X utf8 -m pytest tests\\test_skills_catalog.py::TestGeneratedSkillsIndex
• git diff --cached --check

Video/Screenshots

N/A - markdown skill and catalog update only.

Notes

sync_extensions.py catalog coverage --check reports existing non-blocking coverage warnings for ./plugins/issue-duplicate-checker, ./skills/qa-changes, and ./skills/release-notes.

I also tried running tests/test_skills_catalog.py tests/test_skills_have_readme.py together on Windows. The README tests passed, but one existing test_skills_catalog.py fixture failed because Node ESM cannot import a generated temporary module using a raw C:\\...\\output.js Windows path string. The generated-skills-index subset that validates this PR's checked-in skills/index.js passed.