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/models/copilot-context.ts

@@ -203,6 +203,18 @@ export class CopilotContextModel extends BaseModel {
     return Prisma.join(groups.map(row => Prisma.sql`(${Prisma.join(row)})`));
   }
 
+  async getFileContent(
+    contextId: string,
+    fileId: string,
+    chunk?: number
+  ): Promise<string | undefined> {
+    const file = await this.db.aiContextEmbedding.findMany({
+      where: { contextId, fileId, chunk },
+      select: { content: true },
+      orderBy: { chunk: 'asc' },
+    });
+    return file?.map(f => f.content).join('\n');
+  }
   async insertFileEmbedding(
     contextId: string,
     fileId: string,