docs: Add missing plugins to plugins README

[zeke]

This PR adds documentation for all available plugins in the plugins directory.

Previously, only 4 plugins were documented (agent-sdk-dev, commit-commands, code-review, feature-dev). This PR adds the 5 missing plugins:

• explanatory-output-style - Adds educational insights about implementation choices and codebase patterns
• frontend-design - Creates distinctive, production-grade frontend interfaces
• learning-output-style - Interactive learning mode that requests meaningful code contributions
• pr-review-toolkit - Comprehensive PR review agents for tests, error handling, types, comments, and code quality
• security-guidance - Security reminder hook that warns about potential security issues

Each entry follows the same format as existing plugins, with plugin name, title, description, key features, and use case.

Prompts

Update the plugins README to include all the available plugins. Follow the same format that's used for the existing four in the "Plugins in This Directory" section.