Fix frequent "No tool used" errors by clarifying tool-use rules

src/core/prompts/sections/tool-use.ts

@@ -3,7 +3,7 @@ export function getSharedToolUseSection(): string {
 
 TOOL USE
 
-You have access to a set of tools that are executed upon the user's approval. You can use one tool per message, and will receive the result of that tool use in the user's response. You use tools step-by-step to accomplish a given task, with each tool use informed by the result of the previous tool use.
+You have access to a set of tools that are executed upon the user's approval. You must use exactly one tool per message, and every assistant message must include a tool call. You use tools step-by-step to accomplish a given task, with each tool use informed by the result of the previous tool use.
 
 # Tool Use Formatting