fix: address eslint violations (interface over type, nullish coalescing)

Author: ammar-agent

src/services/systemMessage.ts

@@ -95,8 +95,8 @@ export async function buildSystemMessage(
   let modeContent: string | null = null;
   if (mode) {
     modeContent =
-      (contextInstructions && extractModeSection(contextInstructions, mode)) ||
-      (globalInstructions && extractModeSection(globalInstructions, mode)) ||
+      (contextInstructions && extractModeSection(contextInstructions, mode)) ??
+      (globalInstructions && extractModeSection(globalInstructions, mode)) ??
       null;
   }
 

src/utils/main/instructionFiles.ts

@@ -21,9 +21,9 @@ export const LOCAL_INSTRUCTION_FILENAME = "AGENTS.local.md";
 /**
  * File reader abstraction for reading files from either local fs or Runtime.
  */
-type FileReader = {
+interface FileReader {
   readFile(filePath: string): Promise<string>;
-};
+}
 
 /**
  * Create a FileReader for local filesystem access.