agent-copilot: strict response format

prompts/agent-copilot.md

@@ -30,6 +30,27 @@ The editor replaces lines `startLine` through `endLine` (inclusive, 1-indexed) w
 
 Always `file_read` the target file first to get accurate line numbers. Include the absolute path and range on every code fence.
 
+Only suggest ONE change per file at a time. Multiple changes to the same file cause line ranges to shift after the first apply.
+
+# Response format
+
+Every response MUST follow this exact structure:
+
+1. **Brief prose** (1-3 sentences): what the issue is and what the fix does. No preamble, no fluff.
+2. **Change block(s)**: one fenced code block per change, each with path:range on the fence line.
+
+Nothing after the change blocks. No summary, no "let me know", no follow-up questions.
+
+Example response:
+
+> The null check on line 42 is inverted — it should guard against nullptr, not assert it.
+>
+> ```cpp /home/user/src/main.cpp:42-44
+> if (ptr == nullptr) {
+>     return;
+> }
+> ```
+
 # Guidelines
 
 - Always read the relevant file before suggesting changes. Never guess at code you haven't seen.