Add Claude Code and Codex plugin manifests and MCP registry metadata

[gschier]

Makes Yaak installable by coding agents from the repo, with no registry step:

• /plugin marketplace add mountain-loop/yaak (Claude Code): the use-yaak skill plus an MCP server entry that runs npx -y @yaakapp/cli mcp over stdio.
• codex plugin marketplace add mountain-loop/yaak then codex plugin add yaak@yaak: the skill, with OpenAI listing fields in a top-level plugin.json (Codex ignores them under .codex-plugin).
• npx skills add mountain-loop/yaak --skill use-yaak: the skills CLI now finds the skill through the marketplace file.
• mcpName in @yaakapp/cli and a server.json for the official MCP registry, version-stamped at publish time. Publishing is a manual mcp-publisher step after the next CLI release.
• yaak --help no longer calls a plugin-managed skill install out of date; those keep the unstamped version placeholder and their manager owns updates.

Depends on #752 for yaak mcp; the manifests are inert before it. The plugin root is the CLI crate, which already has the skill in the standard layout, so nothing is copied. Verified with claude plugin validate, a Claude install from a scratch clone, Codex 0.162 with an isolated home, the skills CLI, ajv against each schema, and mcp-publisher validate.

[greptile-apps]

Confidence Score: 5/5

[Low impact] The PR appears safe to merge within its stated scope.

Summary

Adds Claude Code and Codex plugin manifests around the existing Yaak skill.

• Claude Code and Codex can install Yaak's agent plugin from the repo.
• The npm CLI has a versioned listing for the MCP registry.
• yaak --help skips freshness warnings for externally managed skills.

gschier explicitly acknowledges that MCP support depends on #752 and that registry publication remains a manual step after the next CLI release.

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