refactor: improve IDE context format and editor command error handling

- Change IDE context from JSON to plain text format for better LLM comprehension
  - Remove JSON.stringify() and code fences from getIdeContextParts()
  - Use human-readable format: 'Active file:', 'Cursor: line X, character Y'
  - Apply same format to delta updates: 'Files opened:', 'Files closed:', etc.
  - Update all related tests to match new plain text format

- Fix editor command fallback logic in useLaunchEditor
  - Throw clear error when no editor command is available
  - Remove meaningless fallback to last command in list
  - Provide helpful error message with tried commands and solution

packages/cli/src/ui/hooks/useLaunchEditor.ts

@@ -34,11 +34,17 @@ function getExecutableCommand(editorType: EditorType): string {
   const commands =
     process.platform === 'win32' ? commandConfig.win32 : commandConfig.default;
 
-  // Try to find the first available command
   const availableCommand = commands.find((cmd) => commandExists(cmd));
 
-  // Return the first available command, or fall back to the last one in the list
-  return availableCommand || commands[commands.length - 1];
+  if (!availableCommand) {
+    throw new Error(
+      `No available editor command found for ${editorType}. ` +
+        `Tried: ${commands.join(', ')}. ` +
+        `Please install one of these editors or set a different preferredEditor in settings.`,
+    );
+  }
+
+  return availableCommand;
 }
 
 /**