Create VS Code extension for AgentUnit configs

[aviralgarg05]

Summary

Create a VS Code extension that provides syntax highlighting and validation for AgentUnit YAML configs.

Tasks

• Set up VS Code extension scaffolding.
• Add JSON schema for agentunit.yaml files.
• Provide hover documentation for config keys.
• Add snippets for common patterns.

Acceptance criteria

• Extension installable from VSIX.
• YAML validation shows errors inline.
• Autocomplete works for known keys.