feat: Add admin AI assistant with chat interface

[timgl]

Summary

Adds a full-featured AI assistant for admin users with:

• Chat interface on homepage with sidebar for conversation history
• URL-based routing for shareable conversation links (?c=<conversationId>)
• Multi-step agentic tool execution using Vercel AI SDK v6
• Tools for employee data, compensation, and org structure queries
• Auto-create conversations on page load to ensure valid transport
• Database schema updates to support conversations without agents

Implementation Details

• Uses @ai-sdk/react with DefaultChatTransport for streaming chat
• Uses stopWhen: stepCountIs(10) for multi-step tool execution (not maxSteps)
• Server responds with toUIMessageStreamResponse() for proper UIMessage format
• Chat sidebar shows conversation history with delete functionality
• Lazy conversation creation - only creates DB record when admin visits homepage

Tools Available

• readEmployees - Read employee data
• readSalaryHistory - Read salary history
• readProposedHires - Read proposed hires
• readOrgStructure - Read organizational structure
• readCompensationBenchmarks - Read compensation benchmarks

Database Changes

• Made agentId nullable in AgentConversation table (migration included)
• Conversations can exist without being tied to a specific agent record

Testing

Tested locally with multiple conversations, tool execution, and chat history navigation.