docs(plugin-dev): README references stale marketplace name

Summary

plugins/plugin-dev/README.md still tells users to install from plugin-dev@claude-code-marketplace, while the repository's bundled marketplace metadata includes plugin-dev and other bundled plugin docs use the official marketplace naming pattern such as @claude-plugins-official.

Current text

Install from claude-code-marketplace:

/plugin install plugin-dev@claude-code-marketplace

The same README later says the plugin is part of claude-code-marketplace.

Why this is confusing

A user following the README can reasonably try to install from a marketplace name that does not match the rest of the bundled plugin docs. For example, plugins/security-guidance/README.md documents installation as:

/plugin install security-guidance@claude-plugins-official

Suggested fix

Update the plugin-dev README install and contribution wording so it matches the bundled official plugin marketplace naming used elsewhere in the repo.

Scope

Docs-only, likely limited to plugins/plugin-dev/README.md.