Add scenario tagging and filtering

[aviralgarg05]

Summary

Implement scenario tagging and filtering for selective evaluation runs.

Tasks

• Add tags field to Scenario definition.
• Implement --tags / --exclude-tags CLI filters.
• Support tag expressions (e.g., --tags "smoke and not slow").
• Document tagging in scenario docs.

Acceptance criteria

• agentunit eval scenarios/ --tags smoke runs only tagged scenarios.
• Tag expressions with AND/OR/NOT work.
• Untagged scenarios have sensible default behavior.