Implement new create agent sheet (support creating & editing)

[bracesproul]

Depends on #386 #387

Should use the shadcn sheet component

Not all design screenshots have the proper buttons, so ensure to include:

• Cancel + Create buttons in the top right. Create should be disabled when creating until all required fields have been set
• Back / Continue buttons in bottom left. Hide the continue and only show back when they're on the final stem (system prompt)

Need to implement the new designs for creating/editing agents.

1. Configure

2. Triggers

NEED UPDATED DESIGNS

3. Tools

4. Sub-agents

NEED DESIGNS

5. System prompt