feat: ship as a Claude Code plugin marketplace

[MickaelV0]

Summary

• Adds .claude-plugin/marketplace.json and .claude-plugin/plugin.json so this repo doubles as a Claude Code plugin marketplace.
• Users can install the existing ccc skill from inside Claude Code with two commands, alongside the existing npx skills add flow.
• Zero file moves — skills/ccc/SKILL.md is auto-discovered as the plugin's skill; the npx skills add cocoindex-io/cocoindex-code workflow keeps working exactly as before.

Why

Coding agents that natively understand Claude Code plugins (Claude Code itself, and any forks/clients that adopt the schema) get a one-step install path with version pinning, automatic updates via /plugin marketplace update, and discoverability via /plugin install. This complements the existing skill / MCP install paths rather than replacing either.

What changed

File	Change
.claude-plugin/marketplace.json	new — single-plugin marketplace pointing at the repo root
.claude-plugin/plugin.json	new — plugin manifest (name, version, description, author, license, keywords)
README.md	+11 lines — adds a #### Claude Code plugin marketplace subsection under existing "Skill (Recommended)"

How to verify

While this PR is in review, the same manifest is live on the Roxabi-org fork — Claude Code users can already install it with:

/plugin marketplace add Roxabi/cocoindex-code
/plugin install cocoindex-code@cocoindex-code

After merge, the canonical install path becomes:

/plugin marketplace add cocoindex-io/cocoindex-code
/plugin install cocoindex-code@cocoindex-code

In both cases the skill becomes available as /cocoindex-code:ccc (plugin-namespaced) and the agent's automatic search behavior works as documented in skills/ccc/SKILL.md.

Compatibility

• ✅ Existing npx skills add cocoindex-io/cocoindex-code flow unaffected
• ✅ MCP server flow (ccc mcp) unaffected
• ✅ No changes to source code, package, CLI, or tests
• ✅ Apache-2.0 license and authorship metadata preserved in plugin manifest

Open questions for maintainers

• Happy to adjust the marketplace name / plugin name if you'd prefer a different identifier (e.g. ccc vs cocoindex-code).
• email field in owner is set to hi@cocoindex.io — drop or replace if not the right contact.
• version pinned to 1.0.0; if you'd rather rely on git-SHA-based versioning, both version fields can be omitted (per docs).

Drafted as a draft PR — happy to iterate on naming, copy, and metadata before merge.

(Re-opened from Roxabi/cocoindex-code after closing the prior #161 from a personal fork.)