Fix plugin structure for Claude Code skill discovery (#41)

## Summary

- Fix plugin directory structure so Claude Code can discover skills
- Each plugin source now includes `.claude-plugin/plugin.json` and
places `SKILL.md` under `skills/<name>/` subdirectory matching the
expected convention-based layout
- Update CHANGELOG.md

Fixes #40

## Problem

Skills install successfully but are never loaded — `/skills` shows "No
skills found". The root cause is that each plugin source directory has
`SKILL.md` at the root instead of inside a `skills/<name>/`
subdirectory.

## Changes

**Before (broken):**
```
skills/dagster-expert/
├── SKILL.md
└── references/
```

**After (working):**
```
skills/dagster-expert/
├── .claude-plugin/plugin.json
└── skills/dagster-expert/
    ├── SKILL.md
    └── references/
```

## Test plan

- [x] Install original repo as local marketplace → `/skills` shows "No
skills found"
- [x] Install restructured repo as local marketplace → all 3 skills load
correctly in `/skills` and `/context`
- [x] Linters pass (`make lint`)
- [x] CHANGELOG.md updated

## Note

This fix was investigated, developed, and debugged together with [Claude
Code](https://claude.ai/code) (Opus 4.6).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 1oglop1

CHANGELOG.md

@@ -0,0 +1,5 @@
+{
+  "name": "dagster-expert",
+  "description": "Comprehensive Dagster development conventions and best practices",
+  "version": "0.0.8"
+}