refactor: DRY local variant pattern for runtime reads

Extract readFileWithLocalVariantFromRuntime() to match the pattern used
by readFileWithLocalVariant() for local files. This eliminates code
duplication and makes the runtime-based reading follow the same structure.

Author: ammar-agent

src/services/systemMessage.ts

@@ -53,46 +53,86 @@ function getSystemDirectory(): string {
 }
 
 /**
- * Read instruction set from a workspace using the runtime abstraction.
- * This supports both local workspaces and remote SSH workspaces.
+ * Read the first available file from a list using runtime.
  *
  * @param runtime - Runtime instance (may be local or SSH)
- * @param workspacePath - Path to workspace directory
- * @returns Combined instruction content, or null if no base file exists
+ * @param directory - Directory to search in
+ * @param filenames - List of filenames to try, in priority order
+ * @returns Content of the first file found, or null if none exist
  */
-async function readInstructionSetFromRuntime(
+async function readFirstAvailableFileFromRuntime(
   runtime: Runtime,
-  workspacePath: string
+  directory: string,
+  filenames: readonly string[]
 ): Promise<string | null> {
-  const LOCAL_INSTRUCTION_FILENAME = "AGENTS.local.md";
-
-  // Try to read base instruction file
-  let baseContent: string | null = null;
-  for (const filename of INSTRUCTION_FILE_NAMES) {
+  for (const filename of filenames) {
     try {
-      const filePath = path.join(workspacePath, filename);
-      baseContent = await readFileString(runtime, filePath);
-      break; // Found one, stop searching
+      const filePath = path.join(directory, filename);
+      return await readFileString(runtime, filePath);
     } catch {
       // File doesn't exist or can't be read, try next
       continue;
     }
   }
+  return null;
+}
+
+/**
+ * Read a file with optional local variant using runtime.
+ * Follows the same pattern as readFileWithLocalVariant but uses Runtime.
+ *
+ * @param runtime - Runtime instance (may be local or SSH)
+ * @param directory - Directory to search
+ * @param baseFilenames - Base filenames to try in priority order
+ * @param localFilename - Optional local filename to append if present
+ * @returns Combined content or null if no base file exists
+ */
+async function readFileWithLocalVariantFromRuntime(
+  runtime: Runtime,
+  directory: string,
+  baseFilenames: readonly string[],
+  localFilename?: string
+): Promise<string | null> {
+  const baseContent = await readFirstAvailableFileFromRuntime(runtime, directory, baseFilenames);
 
   if (!baseContent) {
     return null;
   }
 
-  // Try to read local variant
+  if (!localFilename) {
+    return baseContent;
+  }
+
   try {
-    const localFilePath = path.join(workspacePath, LOCAL_INSTRUCTION_FILENAME);
+    const localFilePath = path.join(directory, localFilename);
     const localContent = await readFileString(runtime, localFilePath);
     return `${baseContent}\n\n${localContent}`;
   } catch {
     return baseContent;
   }
 }
 
+/**
+ * Read instruction set from a workspace using the runtime abstraction.
+ * This supports both local workspaces and remote SSH workspaces.
+ *
+ * @param runtime - Runtime instance (may be local or SSH)
+ * @param workspacePath - Path to workspace directory
+ * @returns Combined instruction content, or null if no base file exists
+ */
+async function readInstructionSetFromRuntime(
+  runtime: Runtime,
+  workspacePath: string
+): Promise<string | null> {
+  const LOCAL_INSTRUCTION_FILENAME = "AGENTS.local.md";
+  return readFileWithLocalVariantFromRuntime(
+    runtime,
+    workspacePath,
+    INSTRUCTION_FILE_NAMES,
+    LOCAL_INSTRUCTION_FILENAME
+  );
+}
+
 /**
  * Builds a system message for the AI model by combining multiple instruction sources.
  *