Document Agent evaluations, versions, and exports

[biswaroop1547]

Summary

• add end-user guides for Agent evaluations, version review and rollback, and Agent export/import/share
• explain how to start a new Agent chat and what context it receives
• add an Open architecture feature page covering project, file, Agent, conversation, audit, usage, and Skills export boundaries
• keep Agents and Features navigation alphabetical and cross-link the API version reference

Checks

• npm run build
• npm run types:check
• cspell "**/*.mdx" --no-progress --no-summary --show-context
• internal links in changed pages resolved locally