feat: add Claude Code plugin

[myandrienko]

Makes this repo installable as a real Claude Code plugin via .claude-plugin/plugin.json. Expose only /stream to the user; the three sub-skills are agent-invokable.

• .claude-plugin/plugin.json: name stream, metadata mirrors .cursor-plugin/plugin.json (description, version, author, repo, keywords). Default skills/ directory is auto-discovered — no skills field needed.
• stream-cli, stream-docs, stream-builder get user-invocable: false, hiding them from the slash menu. The router still hands off to them by name via the Skill tool.
• stream (router) keeps default invocation. Bare /stream registers unprefixed because each SKILL.md sets name: in frontmatter.
• Cursor manifest left as-is — Cursor has no user-invocable equivalent and routes by description matching, which already produces the same shape.