feat: add agentskill-learn skill for 110K+ skill discovery

[romainsimon]

What

Adds an agentskill-learn skill that connects OpenHands to the agentskill.sh registry, giving users access to 110,000+ cross-platform AI agent skills.

Why

OpenHands users currently need to manually find and import skills. This skill adds a built-in discovery layer that lets agents search, install, and manage skills from the largest open skills registry.

The skill is a lightweight bootstrap (~65 lines) that fetches the full /learn skill from agentskill-sh/learn on first use via the existing add-skill mechanism. This means:

• No maintenance burden: the bootstrap is stable and rarely changes
• Always up-to-date: the real skill is fetched fresh from GitHub
• No duplication: skill logic lives in one canonical repo

Changes

• skills/agentskill-learn/SKILL.md: Bootstrap skill with quick-start API docs and setup instructions
• marketplaces/default.json: Added marketplace entry under "productivity" category

How Tested

• Verified SKILL.md follows the OpenHands skill format (YAML frontmatter with name, description, triggers)
• Verified marketplace entry follows existing patterns (source, category, keywords)
• API endpoints tested: search, install, and trending all return valid JSON

[enyst]

Hey @romainsimon, thank you for this !

I think we should connect agentskills.sh, I have a two little questions:

• I see that the learn skill uses AskUserQuestion, which afaik is a Claude Code tool. OpenHands doesn't define such tool. We have a way to "ask question" the other way around: the user can ask a question about the conversation which is outside agent's history, and I don't think that helps here. 😅 WDYT we could do here, adapt the learn skill to other platforms or would you propose the inclusion of some similar AskUserQuestion in OH? (example)
• we are trying to offer the ability for the user to define multiple marketplaces, and in this repo I believe we have already defined more. Could defining the marketplace or something like that be an alternative way to give the agent the ability to use agentskills.sh?

[romainsimon]

Hey @enyst, thanks for the review!

On AskUserQuestion: Good catch — just pushed an update to the learn skill on our end (commit). The /learn skill now detects whether AskUserQuestion is available in the agent's toolset. When it is (Claude Code, Cursor, etc.), it uses interactive buttons. When it's not (OpenHands, Codex, Aider, etc.), it falls back to numbered lists and text-based input. No changes needed on the OpenHands side. Since the bootstrap SKILL.md in this PR fetches the latest version from agentskill.sh on first use, OpenHands users will automatically get the cross-platform version.

Also added OpenHands to the platform detection (.agents/skills/ directory) and platform mapping table.

On marketplace integration: I think both approaches are complementary. The marketplace is great for curated, pre-defined sets of extensions. The /learn skill adds dynamic capabilities on top: searching 110K+ skills by keyword, trending discovery, security scanning, ratings, and mid-session install. These are things a static marketplace JSON can't do. That said, happy to also explore exposing agentskill.sh as a marketplace source if that's useful. Let me know what format OpenHands expects for external marketplaces and I can look into it.