feat(gateway): load conversation history on session resume (M1-004, M1-005) (#301)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/chat/chat.gateway.ts

@@ -14,7 +14,7 @@ import type { AgentSessionEvent } from '@mariozechner/pi-coding-agent';
 import type { Auth } from '@mosaic/auth';
 import type { Brain } from '@mosaic/brain';
 import type { SetThinkingPayload, SlashCommandPayload, SystemReloadPayload } from '@mosaic/types';
-import { AgentService } from '../agent/agent.service.js';
+import { AgentService, type ConversationHistoryMessage } from '../agent/agent.service.js';
 import { AUTH } from '../auth/auth.tokens.js';
 import { BRAIN } from '../brain/brain.tokens.js';
 import { CommandRegistryService } from '../commands/command-registry.service.js';
@@ -100,12 +100,22 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     try {
       let agentSession = this.agentService.getSession(conversationId);
       if (!agentSession) {
+        // When resuming an existing conversation, load prior messages to inject as context (M1-004)
+        const conversationHistory = await this.loadConversationHistory(conversationId, userId);
+
         agentSession = await this.agentService.createSession(conversationId, {
           provider: data.provider,
           modelId: data.modelId,
           agentConfigId: data.agentId,
           userId,
+          conversationHistory: conversationHistory.length > 0 ? conversationHistory : undefined,
         });
+
+        if (conversationHistory.length > 0) {
+          this.logger.log(
+            `Loaded ${conversationHistory.length} prior messages for conversation=${conversationId}`,
+          );
+        }
       }
     } catch (err) {
       this.logger.error(
@@ -275,6 +285,35 @@ export class ChatGateway implements OnGatewayInit, OnGatewayConnection, OnGatewa
     }
   }
 
+  /**
+   * Load prior conversation messages from DB for context injection on session resume (M1-004).
+   * Returns an empty array when no history exists, the conversation is not owned by the user,
+   * or userId is not provided.
+   */
+  private async loadConversationHistory(
+    conversationId: string,
+    userId: string | undefined,
+  ): Promise<ConversationHistoryMessage[]> {
+    if (!userId) return [];
+
+    try {
+      const messages = await this.brain.conversations.findMessages(conversationId, userId);
+      if (messages.length === 0) return [];
+
+      return messages.map((msg) => ({
+        role: msg.role as 'user' | 'assistant' | 'system',
+        content: msg.content,
+        createdAt: msg.createdAt,
+      }));
+    } catch (err) {
+      this.logger.error(
+        `Failed to load conversation history for conversation=${conversationId}`,
+        err instanceof Error ? err.stack : String(err),
+      );
+      return [];
+    }
+  }
+
   private relayEvent(client: Socket, conversationId: string, event: AgentSessionEvent): void {
     if (!client.connected) {
       this.logger.warn(