Remove collections folder and migrate remaining references to plugins

- Remove rug-agentic-workflow collection files (already migrated to plugin)
- Remove collections.instructions.md (no longer applicable)
- Remove suggest-awesome-github-copilot-collections prompt and plugin command
- Update awesome-copilot plugin.json to reference skills prompt instead of collections
- Update contributor-report.mjs to track plugins instead of collections
- Remove unused collection constants from constants.mjs
- Update rug-agentic-workflow plugin.json with tags and items from collection
- Regenerate README docs and marketplace.json

instructions/collections.instructions.md

@@ -1,54 +0,0 @@
----
-description: 'Guidelines for creating and managing awesome-copilot collections'
-applyTo: 'collections/*.collection.yml'
----
-
-# Collections Development
-
-## Collection Instructions
-
-When working with collections in the awesome-copilot repository:
-
-- Always validate collections using `node validate-collections.js` before committing
-- Follow the established YAML schema for collection manifests
-- Reference only existing files in the repository
-- Use descriptive collection IDs with lowercase letters, numbers, and hyphens
-- Keep collections focused on specific workflows or themes
-- Test that all referenced items work well together
-
-## Collection Structure
-
-- **Required fields**: id, name, description, items
-- **Optional fields**: tags, display
-- **Item requirements**: path must exist, kind must match file extension
-- **Display options**: ordering (alpha/manual), show_badge (true/false)
-
-## Validation Rules
-
-- Collection IDs must be unique across all collections
-- File paths must exist and match the item kind
-- Tags must use lowercase letters, numbers, and hyphens only
-- Collections must contain 1-50 items
-- Descriptions must be 1-500 characters
-
-## Best Practices
-
-- Group 3-10 related items for optimal usability  
-- Use clear, descriptive names and descriptions
-- Add relevant tags for discoverability
-- Test the complete workflow the collection enables
-- Ensure items complement each other effectively
-
-## File Organization
-
-- Collections don't require file reorganization
-- Items can be located anywhere in the repository
-- Use relative paths from repository root
-- Maintain existing directory structure (prompts/, instructions/, agents/)
-
-## Generation Process
-
-- Collections automatically generate README files via `npm start`
-- Individual collection pages are created in collections/ directory
-- Main collections overview is generated as README.collections.md
-- VS Code install badges are automatically created for each item