Implement Claude Plugin Marketplace structure

## Overview

Implement a structured plugin marketplace for Claude Code plugins, served at `ai.arusty.dev/plugins`.

## Current State

- Has `.claude-plugin/` directory with `marketplace.json` (empty) and a README outlining intended structure
- Has `legacy/plugins/` with **50+ plugin directories** already structured with `agents/`, `commands/`, `skills/`
- **Missing**: Individual `plugin.json` manifests in each plugin

## Gap Analysis

| Requirement | Current State | Action Needed |
|-------------|---------------|---------------|
| `plugin.json` manifest | Missing | Add to each plugin |
| Directory structure | ✅ Already correct | None |
| Commands in `commands/` | ✅ Present | None |
| Agents in `agents/` | ✅ Present | None |
| Skills in `skills/` | ✅ Present | None |
| Marketplace registry | Empty `marketplace.json` | Define schema, populate |

## Recommended Structure

```
arustydev/ai/
├── .claude-plugin/
│ ├── marketplace.json # Registry of all available plugins
│ └── README.md
├── plugins/ # Move from legacy/, add manifests
│ ├── backend-development/
│ │ ├── .claude-plugin/
│ │ │ └── plugin.json # Add this
│ │ ├── agents/
│ │ ├── commands/
│ │ └── skills/
│ └── cicd-automation/
│ └── ...
└── components/ # Atomic pieces (current structure)
├── skills/
├── commands/
└── agents/
```

## marketplace.json Schema

```json
{
"version": "1.0.0",
"plugins": [
{
"name": "backend-development",
"description": "Backend development tools",
"version": "1.0.0",
"path": "plugins/backend-development",
"tags": ["backend", "api", "architecture"]
}
]
}
```

## plugin.json Schema (per plugin)

```json
{
"name": "backend-development",
"version": "1.0.0",
"description": "Tools for backend API development",
"author": "aRustyDev",
"tags": ["backend", "api", "rest", "graphql"],
"components": {
"agents": ["api-architect"],
"commands": ["generate-endpoint", "scaffold-service"],
"skills": ["openapi-spec", "database-schema"]
},
"dependencies": []
}
```

## Implementation Tasks

- [ ] Define `plugin.json` schema
- [ ] Define `marketplace.json` schema
- [ ] Move plugins from `legacy/plugins/` to `plugins/`
- [ ] Add `plugin.json` manifest to each plugin (50+ plugins)
- [ ] Populate `marketplace.json` registry
- [ ] Create script to validate plugin structure
- [ ] Set up `ai.arusty.dev/plugins` endpoint
- [ ] Document plugin installation workflow

## Notes

- Marketplace will be served at `ai.arusty.dev/plugins`
- Components in `components/` are atomic building blocks
- Plugins in `plugins/` are bundles that combine components
- Users can install individual components or full plugins

## Related

This issue was created as part of the repository ecosystem strategy discussion.
See: `.ai/plans/repository-ecosystem-strategy.md`