Add Claude Code plugin marketplace support

[fabian-hiller]

This makes the repository installable as a Claude Code plugin marketplace. The previous manifest.json was not a format Claude Code recognizes, so I replaced it with the expected marketplace.json catalog and added a plugin.json manifest to each skill folder. The SKILL.md files stay where they are, so nothing changes for the Agent Skills standard.

Valibot and Formisch are exposed as two separate plugins, so users can install just the one they need:

/plugin marketplace add open-circle/agent-skills
/plugin install valibot@open-circle
/plugin install formisch@open-circle

All three manifests pass claude plugin validate --strict, and I tested the full marketplace add and install flow locally. The installed valibot plugin loads its skill with a footprint of about 61 tokens per session.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Expose SKILL.md from each plugin root

For either documented install command, these source values make skills/{valibot,formisch} the plugin roots, but the official plugin directory structure discovers skills under <plugin-root>/skills/<skill>/SKILL.md. A repo-wide target-tree search shows only <plugin-root>/SKILL.md, and neither new manifest declares a custom skill path, so both installed plugins have no discoverable skill despite their manifests validating; expose the existing root files through valid plugin skill paths while preserving the required Agent Skills layout.

AGENTS.md reference: AGENTS.md:L16-L24

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