feat: add Claude Code plugin marketplace

[ankitranjan7]

Summary

Adds a Claude Code plugin and marketplace manifest for Webcmd so Claude Code users can install agent support through Claude's native plugin system instead of the generic npx skills add path.

This PR is intentionally stacked on codex/single-discovery-skill because the Claude plugin should expose the single skills/webcmd discovery skill, while detailed workflow instructions continue to come from the installed webcmd CLI via webcmd skills list and webcmd skills get.

Why this change

Claude Code has its own first-class plugin marketplace flow:

/plugin marketplace add agentrhq/webcmd
/plugin install webcmd@webcmd

That is the right Claude Code equivalent of the Codex plugin path. A user who is only using Claude Code should not need to install Webcmd's generic Agent Skills package separately. They should install the CLI once with npm, then install the Claude Code plugin for agent instructions.

This also covers Claude Desktop's Code tab for local and SSH sessions, because that surface uses Claude Code plugins. We are not adding separate Claude Desktop Chat or Cowork installables in this PR because those surfaces are connector/MCP/desktop-extension oriented and would require a different product surface.

What changed

• Adds .claude-plugin/plugin.json with Webcmd plugin metadata and skills: "./skills/".
• Adds .claude-plugin/marketplace.json so the repository can be added as a Claude Code marketplace.
• Includes .claude-plugin/** in the npm package files so published artifacts preserve the plugin metadata.

Best practices followed

• Uses Claude Code's native plugin marketplace instead of asking Claude Code users to install generic skills manually.
• Keeps the plugin minimal: one discovery skill, no speculative hooks, agents, MCP servers, or desktop extensions.
• Keeps plugin and marketplace metadata explicit: stable plugin name, display name, description, owner, repository, license, keywords, and category.
• Pins plugin version to the package version so release/update semantics are easy to reason about.
• Keeps this PR separate from the Codex plugin PR so each agent-specific packaging surface can be reviewed independently.

Validation

claude plugin validate .
# ✔ Validation passed

node -e "for (const f of ['.claude-plugin/plugin.json','.claude-plugin/marketplace.json','package.json']) JSON.parse(require('fs').readFileSync(f,'utf8')); console.log('json ok')"
# json ok

npm pack --dry-run
# included .claude-plugin/marketplace.json and .claude-plugin/plugin.json

Follow-up

After this stack lands, the README/docs PR points Claude Code users to this plugin-native install flow. If Webcmd later needs Claude Chat or Claude Cowork support, that should be a separate MCP/desktop-extension design rather than extending this Claude Code plugin.