feat(server): add hints for context files (#13444)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Attachments (files) are now included in the conversation context,
allowing users to reference files during chat sessions.
* Added a new "blobRead" tool enabling secure, permission-checked
reading of attachment content in chat sessions.

* **Improvements**
* Enhanced chat session preparation to always include relevant context
files.
* System messages now clearly display attached files and selected
content only when available, improving context clarity for users.
* Updated tool-calling guidelines to ensure user workspace is searched
even when attachment content suffices.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/copilot/controller.ts

@@ -45,6 +45,7 @@ import {
   UnsplashIsNotConfigured,
 } from '../../base';
 import { CurrentUser, Public } from '../../core/auth';
+import { CopilotContextService } from './context';
 import {
   CopilotProvider,
   CopilotProviderFactory,
@@ -75,6 +76,7 @@ export class CopilotController implements BeforeApplicationShutdown {
   constructor(
     private readonly config: Config,
     private readonly chatSession: ChatSessionService,
+    private readonly context: CopilotContextService,
     private readonly provider: CopilotProviderFactory,
     private readonly workflow: CopilotWorkflowService,
     private readonly storage: CopilotStorage
@@ -204,14 +206,24 @@ export class CopilotController implements BeforeApplicationShutdown {
       retry
     );
 
-    if (latestMessage) {
-      params = Object.assign({}, params, latestMessage.params, {
-        content: latestMessage.content,
-        attachments: latestMessage.attachments,
-      });
-    }
+    const context = await this.context.getBySessionId(sessionId);
+    const contextParams =
+      Array.isArray(context?.files) && context.files.length > 0
+        ? { contextFiles: context.files }
+        : {};
+    const lastParams = latestMessage
+      ? {
+          ...latestMessage.params,
+          content: latestMessage.content,
+          attachments: latestMessage.attachments,
+        }
+      : {};
 
-    const finalMessage = session.finish(params);
+    const finalMessage = session.finish({
+      ...params,
+      ...lastParams,
+      ...contextParams,
+    });
 
     return {
       provider,