[Feature] Implement AI assistants (Q4 2025)

As a user, I want to add specialized AI assistants to my workspace, so that I can have expert help for specific tasks and domains.

As a user, I want to create custom AI assistants with specific knowledge and instructions, so that I can automate specialized workflows.

As a user, I want to add pre-configured AI assistants, so that I can quickly access specialized expertise for coding, writing, analysis, and other tasks.

As a team member, I want to add AI assistants to our workspace, so that our team has consistent, specialized AI help for our specific needs.