feat: US-002 - Modify Tool interface to return *ToolResult

- Update all Tool implementations to return *ToolResult instead of (string, error)
- ShellTool: returns UserResult for command output, ErrorResult for failures
- SpawnTool: returns NewToolResult on success, ErrorResult on failure
- WebTool: returns ToolResult with ForUser=content, ForLLM=summary
- EditTool: returns SilentResult for silent edits, ErrorResult on failure
- FilesystemTool: returns SilentResult/NewToolResult for operations, ErrorResult on failure
- Temporarily disable cronTool in main.go (will be re-enabled in US-016)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/tools/message.go

@@ -55,10 +55,10 @@ func (t *MessageTool) SetSendCallback(callback SendCallback) {
 	t.sendCallback = callback
 }
 
-func (t *MessageTool) Execute(ctx context.Context, args map[string]interface{}) (string, error) {
+func (t *MessageTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
 	content, ok := args["content"].(string)
 	if !ok {
-		return "", fmt.Errorf("content is required")
+		return &ToolResult{ForLLM: "content is required", IsError: true}
 	}
 
 	channel, _ := args["channel"].(string)
@@ -72,16 +72,24 @@ func (t *MessageTool) Execute(ctx context.Context, args map[string]interface{})
 	}
 
 	if channel == "" || chatID == "" {
-		return "Error: No target channel/chat specified", nil
+		return &ToolResult{ForLLM: "No target channel/chat specified", IsError: true}
 	}
 
 	if t.sendCallback == nil {
-		return "Error: Message sending not configured", nil
+		return &ToolResult{ForLLM: "Message sending not configured", IsError: true}
 	}
 
 	if err := t.sendCallback(channel, chatID, content); err != nil {
-		return fmt.Sprintf("Error sending message: %v", err), nil
+		return &ToolResult{
+			ForLLM:  fmt.Sprintf("sending message: %v", err),
+			IsError: true,
+			Err:     err,
+		}
 	}
 
-	return fmt.Sprintf("Message sent to %s:%s", channel, chatID), nil
+	// Silent: user already received the message directly
+	return &ToolResult{
+		ForLLM: fmt.Sprintf("Message sent to %s:%s", channel, chatID),
+		Silent:  true,
+	}
 }