Add support for multiple git repositories in prompt sync extension

[Copilot]

This PR implements support for syncing prompts from multiple Git repositories simultaneously, addressing the need for users who collaborate across multiple projects or maintain prompt collections in different repositories.

Key Changes

🔧 Configuration Schema Updates

• Added promptsSync.repositories array setting to support multiple repository URLs
• Maintained backward compatibility with existing promptsSync.repository single repository setting
• Updated VS Code extension manifest with proper JSON schema validation

🚀 Enhanced Sync Logic

• Multi-Repository Processing: The sync manager now iterates through all configured repositories sequentially
• Individual Error Handling: Each repository is processed independently - failures in one repository don't prevent others from syncing
• Aggregated Results: Combines results from all repositories to provide comprehensive sync statistics

📊 Improved User Experience

• Partial Success Notifications: New notification system handles scenarios where some repositories succeed while others fail
• Detailed Status Display: Shows repository count and individual repository URLs in the status view
• Enhanced Error Reporting: Repository-specific error messages help users troubleshoot issues

🔄 Backward Compatibility

The implementation ensures existing configurations continue working without changes:

• If promptsSync.repositories is empty or undefined, falls back to promptsSync.repository
• All existing settings and behaviors remain unchanged
• No breaking changes for current users

Example Configuration

{
  "promptsSync.repositories": [
    "https://github.com/your-org/prompts-main",
    "https://github.com/your-org/prompts-experimental", 
    "https://github.com/another-org/shared-prompts"
  ]
}

Error Handling

The extension gracefully handles various scenarios:

• Complete Success: All repositories sync successfully
• Partial Success: Some repositories succeed, others fail (shows warning with details)
• Complete Failure: All repositories fail (shows error notification)

Each scenario provides appropriate user feedback with actionable next steps.

Fixes #1.

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[Copilot]

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[jeanplevesque]

@NathBo470 Please add screenshots of how it looks in VS Code to setup the multi-repos.

[MatFillion]

Looks like this @NathBo470 @jeanplevesque