docs(plugin-structure): remove extraneous README.md from skill (#87)

## Summary

Remove non-conformant `README.md` from the `plugin-structure` skill
directory.

## Problem

Fixes #84

The `plugins/plugin-dev/skills/plugin-structure/README.md` file doesn't
conform to official Claude Code skill structure. Per the [skills
documentation](https://code.claude.com/docs/en/skills.md), skill
directories should only contain:
- `SKILL.md` (required)
- `references/` (optional)
- `examples/` (optional)
- `scripts/` (optional)
- `assets/` (optional)

## Solution

Delete the extraneous README.md file. All relevant information is
already available in `SKILL.md`.

### Alternatives Considered

- **Keep and rename to something valid**: Rejected - the content is
redundant with SKILL.md
- **Move to assets/**: Rejected - not an asset, and still non-conformant
metadata

## Changes

- Removed `plugins/plugin-dev/skills/plugin-structure/README.md` (108
lines)

## Testing

- [x] Verified skill directory structure matches official spec
- [x] Linting passes (markdownlint)

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

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

Author: sjnims