feat(31-02): create chat types for conversation interface

- ChatRole, ChatMessage, ConversationState types
- ChatRequest for /api/chat endpoint
- Minimal types (Vercel AI SDK provides its own)

src/types/chat.ts

@@ -0,0 +1,39 @@
+/**
+ * Chat Types
+ *
+ * Types for the conversational workflow planning interface.
+ */
+
+/** Role in conversation */
+export type ChatRole = 'user' | 'assistant' | 'system';
+
+/** Single message in conversation */
+export interface ChatMessage {
+  /** Unique message ID */
+  id: string;
+  /** Who sent the message */
+  role: ChatRole;
+  /** Message content (markdown supported) */
+  content: string;
+  /** When message was created */
+  createdAt: Date;
+}
+
+/** State of the chat conversation */
+export interface ConversationState {
+  /** All messages in the conversation */
+  messages: ChatMessage[];
+  /** Whether AI is currently responding */
+  isLoading: boolean;
+  /** Current error if any */
+  error: string | null;
+}
+
+/** Request body for /api/chat */
+export interface ChatRequest {
+  /** Message history */
+  messages: Array<{
+    role: ChatRole;
+    content: string;
+  }>;
+}