Reapply "Fix Assist to use mailbox system instead of direct 'in' file writes"

This reverts commit 8a9ce2c9e1.

cmd/Assist/main.go

@@ -3,6 +3,7 @@ package main
 
 import (
 	"anvillm/internal/sandbox"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"log"
@@ -354,6 +355,42 @@ func sendPrompt(id, prompt string) error {
 	if !isConnected() {
 		return fmt.Errorf("not connected to anvilsrv")
 	}
+	
+	// Create message JSON
+	msg := map[string]interface{}{
+		"to":      id,
+		"type":    "PROMPT",
+		"subject": "User prompt",
+		"body":    prompt,
+	}
+	msgJSON, err := json.Marshal(msg)
+	if err != nil {
+		return fmt.Errorf("failed to marshal message: %w", err)
+	}
+	
+	// Write to user outbox
+	timestamp := time.Now().Unix()
+	filename := fmt.Sprintf("msg-%d.json", timestamp)
+	path := filepath.Join("user/outbox", filename)
+	
+	fid, err := fs.Create(path, plan9.OWRITE, 0644)
+	if err != nil {
+		return fmt.Errorf("failed to create message file: %w", err)
+	}
+	defer fid.Close()
+	
+	_, err = fid.Write(msgJSON)
+	if err != nil {
+		return fmt.Errorf("failed to write message: %w", err)
+	}
+	
+	return nil
+}
+
+func sendPromptDirect(id, prompt string) error {
+	if !isConnected() {
+		return fmt.Errorf("not connected to anvilsrv")
+	}
 	path := filepath.Join(id, "in")
 	fid, err := fs.Open(path, plan9.OWRITE)
 	if err != nil {
@@ -570,8 +607,15 @@ func handlePromptWindow(w *acme.Win, sess *SessionInfo) {
 			}
 			prompt := strings.TrimSpace(string(body))
 			if prompt != "" {
-				if err := sendPrompt(sess.ID, prompt); err != nil {
-					fmt.Fprintf(os.Stderr, "Failed to send prompt: %v\n", err)
+				var sendErr error
+				if cmd == "Prompt" {
+					sendErr = sendPromptDirect(sess.ID, prompt)
+				} else {
+					sendErr = sendPrompt(sess.ID, prompt)
+				}
+				if sendErr != nil {
+					fmt.Fprintf(os.Stderr, "Failed to send %s: %v\n", strings.ToLower(cmd), sendErr)
+					continue
 				}
 				w.Ctl("delete")
 				return