Implement tree structure groups for hierarchical resource organization

[Copilot]

This PR implements the requested tree structure groups feature that allows organizing instructions, chatmodes, and prompts in a hierarchical structure with group-level enable/disable functionality.

Overview

The feature automatically detects folder-based organization in catalogs and creates collapsible groups in the VS Code tree view. Users can now enable/disable entire groups of related resources with a single click, making it much easier to manage complex catalogs with many resources.

Key Changes

Resource Model Extensions:

• Added groupPath property to Resource interface to track folder-based organization
• New ResourceGroup interface for managing hierarchical groups with state tracking
• Support for nested groups with recursive enable/disable operations

Enhanced Resource Discovery:

• New extractGroupPath() method automatically detects group structure from file paths
• Example: chatmodes/ai-agents/code-assistant.chatmode.md → creates "ai-agents" group
• Works seamlessly with all existing catalog discovery modes (root override, per-category, remote)

Tree Provider Updates:

• CategoryTreeProvider now displays collapsible folder groups with proper state indicators
• Group nodes show activation status: ✅ (fully enabled), ⚠️ (partially enabled), ⏸️ (disabled)
• Resource counts displayed as (active/total) for each group
• Ungrouped resources continue to display normally for backward compatibility

Bulk Operations:

• New commands: copilotCatalog.activateGroup and copilotCatalog.deactivateGroup
• Context menus on group items provide easy access to bulk operations
• Recursive activation/deactivation of all resources in a group and its children

Example Transformation

Before (flat structure):

📁 Chat Modes (0/5)
├── ⏸️ code-assistant
├── ⏸️ documentation-writer
├── ⏸️ testing-assistant
├── ⏸️ release-workflow
└── ⏸️ review-process

After (hierarchical groups):

📁 Chat Modes (1/5)
├── 📁 ai-agents (1/3) ⚠️
│   ├── ✅ code-assistant
│   ├── ⏸️ documentation-writer
│   └── ⏸️ testing-assistant
└── 📁 workflows (0/2) ⏸️
    ├── ⏸️ release-workflow
    └── ⏸️ review-process

Catalog Structure Support

The feature works with existing folder structures:

copilot_catalog/
├── chatmodes/
│   ├── ai-agents/           ← Becomes "ai-agents" group
│   │   ├── code-assistant.chatmode.md
│   │   └── testing-assistant.chatmode.md
│   └── workflows/           ← Becomes "workflows" group
│       └── release-process.chatmode.md
├── instructions/
│   ├── setup/               ← Becomes "setup" group
│   └── advanced/            ← Becomes "advanced" group
└── prompts/
    └── dev/                 ← Becomes "dev" group

Benefits

• Logical Organization: Related resources are visually grouped together
• Efficient Workflow: Enable/disable entire categories of functionality with one click
• Clear Visibility: Immediate understanding of which groups are active/inactive
• Scalable Management: Handle large catalogs with many resources efficiently
• Backward Compatible: Existing flat catalogs continue to work unchanged

Testing

• Unit tests validate group path extraction and state calculation logic
• Integration tests verify hierarchical catalog discovery
• VSIX package built and ready for deployment
• Comprehensive documentation and visual demos created

The feature requires no configuration - groups are automatically created based on existing folder structures in catalogs.

Fixes #10.

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