fix: improve instruction to explicitly require file changes

After reviewing the Auggie SDK source code, isAnswerOnly only controls
which text is returned, NOT whether tools are executed. The real issue
was that the instruction wasn't explicit enough.

Changes:
- Add clear context that this is an AI assistant implementing changes
- Explicitly state 'You MUST actually implement the requested changes'
- Tell Auggie to use file editing tools (str-replace-editor, save-file)
- Clarify NOT to just describe what would be done
- Set isAnswerOnly back to true (correct setting for clean final answer)

This ensures Auggie understands it needs to actually edit files, not
just provide a description of what it would do.

Author: Tulga Tsogtgerel

assistant/src/index.ts

@@ -273,7 +273,10 @@ async function invokeAuggie(context: PRContext): Promise<void> {
   }
 
   // Build context for Auggie
-  const instruction = `${context.commentBody}
+  const instruction = `You are an AI assistant helping to implement code changes in a Pull Request.
+
+## User Request
+${context.commentBody}
 
 ## PR Context
 - **PR #${context.prNumber}**: ${context.title}
@@ -292,7 +295,10 @@ ${context.commentThread.map(c => `**${c.author}** (${c.createdAt}):\n${c.body}`)
 ${context.diff}
 \`\`\`
 
-Please implement the requested changes based on the comment and PR context above.`;
+## Instructions
+You MUST actually implement the requested changes by editing the files. Do NOT just describe what you would do.
+Use the str-replace-editor, save-file, or other file editing tools to make the actual changes to the code.
+After making the changes, provide a brief summary of what you implemented.`;
 
   core.info('📤 Sending request to Auggie...');
 
@@ -310,7 +316,7 @@ Please implement the requested changes based on the comment and PR context above
 
     // Send the instruction to Auggie
     core.info('💬 Sending instruction to Auggie...');
-    const response = await auggie.prompt(instruction, { isAnswerOnly: false });
+    const response = await auggie.prompt(instruction, { isAnswerOnly: true });
 
     core.info('📝 Auggie response:');
     core.info(response);